I'm somewhat confused. Do you already have everything listed built, or are you asking for advice on the entire build with a $2,650 budget? Besides, the SBM builds really aren't that great. If you're asking for advice on the full build, I'll post a better/cheaper one below.
A third GPU isn't going to give you much of a performance gain, so I definitely wouldn't waste any money with that option. If you're looking for more FPS, you need to be getting either dual 5870s or a 5970.
SSDs are nice, but they're really expensive right now. I'd wait for a while before shelling out a lot of money on one.
Here's the better build (I'll check that the combos are still there):
Yes. At stock, the 5970 is the same thing as 5850 CF. However, technically, the 5970 is two 5870 cores on a single card. If you overclock the 5970, you can get that speed back or at least get close to it. So the 5970 is actually more powerful than 5850 CF.
Also, you don't lose an upgrade path right away. By having a single card, you can add more later on. If you start with dual 5850s, adding a third isn't going to do much.
EDIT: I should add that there are other things I'd change in your build too. Like everything except the CPU. The board is overly expensive, the PSU is too large, the RAM isn't likely to be that fast (don't know since you didn't link to it), the HDD is overly expensive, and BR doesn't have a point for computers yet (hold off until a use specific to computers that you need/want shows up or the prices drop significantly).
This was basically the last THW system build. I just changed the MB to take advantage of 6G(and reason for the new HD)
My last pc I built was a Raid0 250x2. Don't know how well it would perform vs. a 6g. Just the Raid0 a pain since I whipe/restart my OS every 6-12months.
I am clueless on sizing the right powersupply. I just heard its nice to pump up the supply so it keeps enough power as it ages and ensure peak power is there when nessecary.
I have the money, but don't want to spend it foolishly either. It doesn't have to be the fastest, but want it to last 3 years before it shows its age. The last one I built is just barely starting to show its own.
2Gig CAS 2 Corsair
Raid 0 250x2 WD 7200
I'm not a fan of SBM builds. The only consideration they make is current performance. The builds aren't actually good ideas if you keep the computer around for more than a year or two.
The SATA III HDDs don't actually offer much performance gain. The reason they're fast is because they use 500 GB platters, unlike previous generations. There are SATA II HDDs that are just as fast and cheaper (namely, the Samsung Spinpoint F3 and Seagate 7200.12). The WD is a touch faster, but it's not noticeable. Besides, you should be running everything off the SSD besides data storage, so you shouldn't run into HDD speed issues.
Before learning what sizes are good for what GPUs, I just used a PSU wattage calculator (there's a link in one of general info threads linked to in the stickies to a really good one). The typical guideline is that for every GPU you add, you need to add 200W to the PSU.
The build above definitely won't show any age for a long time. The 5970 is complete overkill for everything. It's limits are just barely reached when playing Crysis at 5760x1080 (Eyefinity with three 1080p monitors). It crushes everything at resolutions under that. It's typically thought that an upgrade for the 5970 is a good 4-5 years out. Even then, you'd be able to drop a second one into the above build for a much lower price and add a good 2-3 years on to the life of the build.