Get what you need when you need it...
Someone else can correct me if I am wrong but AFAIK, Mac Pro has to be the ECC, Now if I am correct, which I think I am here, than the memory is high priced at both places you listed.
Regarding the ACD's being old and grainy, hmmm older technology yes, grainy I have never seen a grainy ACD, have to remember whats driving the ACD also.
As others have said, get the machine you need now. There will ALWAYS be another better, bigger machine released about three days after the return policy is over. Unless you have a lot bigger bank account than me, I would recommend purchasing what you need now and upgrade as needed.
I have a 2008 Mac Pro (8 core 2.8GHz) with 16GB RAM and an Apple RAID Card running an Apple supplied Seagate 750 boot drive and 3 - 750GB Samsung aftermarket drives. Note: Not all aftermarket memory FB-DIMMs are made the same. I went with OWC memory with their specs and lifetime warranty. Currently, they are sold for $185.99 for 4 modules of 2GB each. You should always have matched pairs in the risers in a Mac Pro for the best performance.
While at the time I purchased the drives from Newegg,
http://www.macsales.com currently has really great prices. Drive/memory prices have DROPPED like a stone in the last few months. Both storage and memory are at crazy low prices right now.
I remember the time when only 1MB used to be $700 USD or more! Today you can find memory in the GB range for less than $25 USD! Do a quick Net search and find the best deals for you.
I also went with an aftermarket Samsung 24" monitor. This box runs my business with a Parallels VM running XP used for the laser print driver. I can run anything you can imagine with Final Cut Pro, FileMaker Advanced, Aperature as well as PhotoShop, Corel Draw and PhotoGrav on the XP side.
I also have a cloned drive in the gun safe should I have a need to boot from a total disaster without spending countless hours on installs and updates.
As you can see, the Mac Pro with the right software and hardware can run virtually anything out there today. As for performance, even in 32-bit, Geekbench runs at 8929!
Don't forget to defragment AND backup the drives either offsite or in a fireproof safe on a regular basis no matter what you get or are running.
If I need more, then I guess Snow Leopard should give an added boost.
Really, I cannot imagine a current Mac Pro, or for that matter, an iMac or Power Book with enough memory not being able to give anyone what they truly need for a LONG time.
Just be smart and purchase the memory, hard drives and monitor third party.