For the most part, that's pretty damn good. I've been looking at a couple of those parts myself for my own rig. Just 2 concerns. 1: i would highly reccomend that you get a better PSU. 550 watts will just barely cut it for this rig and leaves no room for expanding. I would go for atleast 700 watts. 2: you chose to go for DDR2-800 RAM when your mobo supports DDR2-1600 RAM. this isn't a major issue but you will get better performance if you go for the higher clock speed. You might also want to go for a full-tower case instead of a mid-tower case. full-tower cases take up quite a bit more room but in turn you get alot more room on the inside, and with the graphics card you chose, it could get pretty cramped with a mid-tower case.

other than that, great system! as for SLI, they do need to be the same. however the mobo you chose only has one PCI express x16 slot. you also have to make sure the mobo you select supports SLI (obviously this one doesn't because it only has one graphics slot).

Go to your graphics card manufacturer's website and make sure you have the latest drivers. Also be sure that your in-game settings haven't been changed. If you have your resolution set to a higher one than you had before, then that could cause you to get lower FPS rate. Also check your anti-aliasing settings and other things of that nature which require more GPU power. If all the settings are the same as they were before and your drivers are up to date then i'm clueless.

you should check the motherboard to see if there are any blown capacitors on it. capacitors are those things that look like soda cans. normally they should have a flat top but if you see one that's curved up or cracked or one that has a brown-ish substance at the top then that means it's blown. one blown capacitor is enough to kill a motherboard.

that thing is for the most part future-proof. however you may be overdoing it when compared to today's standards. you should try building your own machine.