1) RAID Setup - I wouldn't recommend building/buying an internal RAID setup. There are plenty of inexpensive RAID systems available on the market right now. Sounds like you're looking for 4-TB of storage. I would recommend you buy the standard 640GB internal drive for your boot drive. You install all your applications, etc... on this drive. Then use an external RAID to store data. Here are some suggestions:
G-Technology G-RAID 4TB (FW800 or eSATA - 2-drives - RAID 0) $490
G-Technology G-SPEED eS 4TB (eSATA - 4-drives - RAID-5 - includes PCIe eSATA card) $1190
G-Technology G-SPEED eS PRO 4TB (mini-SAS - 4-drives - RAID 5 - includes PCIe mini-SAS card) $2100
You could also go with an "Apple certified" RAID
Apple Promise SmartStor DS4600 4TB (FW800 only - 4-drives - RAID-5) $795
These are software controlled devices, not hardware. That means they're missing a processor and cache. So they're not capable of handling parity based arrays properly (5/6/50/60), as there's no NVRAM on it to handle the write hole issue associated with parity arrays.
FW800 is slow. eSATA is better, but you still will have a bottleneck at some point.
Ultimately, they have limitations, and aren't the most reliable things out there, so I don't trust them enough to recommend it for a primary array. For backup, they're fine.
Or if you don't need a lot of speed, you could buy a Drobo and have storage that can grow with your needs:
Data Robotics Drobo 4TB (FW800 only - with 4-drives - RAID 5) $680 after mail in rebate
These are also software based, and have the same issues associated with it.
Understand, they're fine for 0/1/10, but that's it. Just don't ever try to use them with parity based arrays, as you will get burnt (it's just a matter of when).
3) BOOT DRIVE - If you can afford it, I would recommend buying a second internal drive and combining them together as a simple RAID. This will make your applications launch much faster. The negative with doing this is that you will have to reinstall ALL the software that comes with your computer. (Snow Leopard, iLife applications, etc...) That would take considerable time, but it might be worth it if you use a lot of different applications throughout your work day.
Theoretically, a stripe set (2x disks) will cut the random access times in half. Unfortunately, this is rare, and the likely improvement is more like a 2ms reduction (i.e. single drive mode = 12ms, and a stripe set of a pair would produce ~10ms; it has to do with file size and distribution).
OS loading is random access, which isn't mechanical's strong suit, striped or not. SSD's rule for random access performance, but it's not cheap.
The Apple sales rep said I'd notice a 7-10% increase in performance with a RAID setup. Compared to the cost of upgrading processor speed, it seemed like the way to go. The only backing up of video files I need to do is done on external hard drives. I work with them when I need to, then I store them away. And I'm pretty confident that they won't feel comfortable setting up an internal RAID. They're recommending I go with Apple's, but I haven't seen one positive post about them. Do I even need a RAID, I wonder...
Apple's card is JUNK. Don't touch it. It's really that bad.
In the other thread, you only asked about a decent card. But what exactly are you doing?
A card can do more than just offer a speed boost. It has to do with recovery in the event of a failure.
A stripe set can provide you with an improved sequential throughput, but if it fails, you're data is gone. Fix the problem, and then restore the data from backup source/s. This takes time. If you can afford to do this, that's fine. If not, you need to consider a different array type (redundancy, and it varies with different levels). Throughput requirements as well as capacity requirements would also be needed to guide you to the right level.
It seems a look at
Wiki's RAID page might be in order, as it does give you an explanation of what the differences are between the levels, as well as other useful information.
Graphics Card. That 4870 option to power both 24" and a GT 120 to run the preview sounds like a good solution. Since I won't do any gaming on this machine, will I notice the difference between the 4870 and GT120 when video editing?
This will be fine from what information you've given.
Striping two mechanical drives won't make the app loading considerably faster. Apps don't come in large sequential files, so a RAID0 would be almost useless.
If you really want to boost your boot drives performance, get a decent SSD.
Nope. OS and applications rely on random access throughputs, not sequential.
Which ever route you choose, there is no reason to re-install the whole system including all applications. There are several cloning applications available that will make a carbon copy of any drive you select. You can then simply replace the old drive with the new one without installing anything again.
I recommend Carbon Copy Cloner for such tasks.
Good advice. I just held off on such things until there's a better understanding of what the OP's needs are (get the right solution).
I wanted to go the SSD route, but the Apple technician I spoke with over the phone said that an SSD would be a mistake due to reliability/longevity issues. He is the one who recommend I go with two standard hard drives and stripe them into a RAID. He said that two mechanical drives striped together as a RAID would be faster than a single SSD drive and more reliable.
I'm just repeating what the Apple tech told me.
BS. Who ever told you this is full of it.
Striping mechanical drives will give you a sequential throughput similar to an SSD, but not random access, which is what the OS and applications rely on as they're large numbers of small files loaded in succession.
I'm not a telepath. It'd probably drive me to the loony bin if I were.

And I'm not far from it as is.
Having said all that, once you buy an apple raid card, the apple support have a section called "enterprise" and you can talk directly to them. They are like super geniuses I guess and will be able to help you set it all up - if you get their card that is...
Areca's aren't that hard to setup either, and the manual is actually decent.
Support is usually through email (I've never made an international call to them, so I've no idea if it's better over the phone), as they're based in Taiwan. Their English can be challenging at times, but they do know what they're doing, as they actually design, manufacture, and test their own gear unlike most other card vendors (CalDigit, Apple, Highpoint for example).