We are quibbling details here.... but here are my thoughts about the HDD plan above.
Put the scratch disk on a 7200rpm drive, if you getting one. Make sure it's the first partition (this puts it, according to the research I did, on the outside of the platter. Seek times are faster here.)
You don't need to back up the scratch disk. It is populated when PS opens and you open files, then the contents are deleted when PS closes.
Make sure you have off-site backups. Since you are not a professional, you can probably get away with swapping the off-site back up weekly or even monthly. However, also have on-site back-ups. They do two different things.
The on-site back up should be updated at least daily. Get Carbon Copy Cloner or SuperDuper! (or similar) and let it run nightly. This creates a bootable backup. Plus, it's just automatic. You don't need to do anything once it's set up. Time machine also works well.
What SD! and CCC do is create a clone of your HDDs. If a drive fails, you buy a new drive and do a "restore". If the computer fails, CCC and SD! can actually be used to "boot" a loaner computer.
Time Machine is useful in that it keeps things up to date in 10 minute chunks (iirc) plus keeps versions of things so you can "undelete" files later on. I don't use TM myself.
You are most likely to need to go back to an on-site daily backup. HDDs fail. Computers break. All the time. Daily backups should be painless and automatic. Also, daily backups are probably only needed to replace the contents of a single HDD that crashed.
But.... occasionally there is the risk of a total system failure. Glass of water into system. Burst pipe in the room above you. (Water can 'run' really far... so the burst pipe can be in the bathroom above and over 3 rooms). Fire. Theft. Earthquake. Tree falling in a storm. This is when you want the offsite backup. You may lose everything you did since the last swap, but you haven't lost everything. I put mine in a safety deposit box. I've read about pro photographers who ship a HDD to friends off-continent.
Last thought. No one here has the foggiest idea when Apple is going to release the next Mac Pro. Or even if there is going to be a next version. Remember that this is a "Rumours" site, not a "News" site. So read up on the buyer's guide. Read some threads about when the next Mac Pro will be released. Some of us have an educated guess - but it's just that - a guess. And the "educated" I just add to make myself feel better.