Video Cards! need help!

AFAIK, you can SLI an 8800GTX made by BFG with an 8800GTX made by eVGA, for example. The model must be the same though (that is, the 8800GTX part). Only variations in manufacturer, clocks, maybe amount of RAM are allowed. For example 8800GTS 320MB + 8800GTS 640MB used to work, but the second card was treated as a 320Mb card. Maybe all this has changed in recent months, I don't know.

I'd suggest you get a GTX 260 and see how it feels. It may be just fine for your needs. Add a second one later if needed. A third one only if you're running a 30" monitor.

What kind of monitor and case and PSU do you have? Speaking of PSU, check out this page.
http://www.slizone.com/object/slizone_build_psu.html

All right. First thing you should know, a $200 video card is usually smarter than two $100 cards in SLI. I suggest you get the best video card you can afford. Make it an nVidia card because your MB is SLI-capable. Get a PSU that can support two cards like the one you're buying, so you have a good upgrade path for later.

There's a 9800 GX2 for $229 at newegg, but it's "recertified" and I don't know how much it can be trusted. Still, compared to $370 for a normal one, it may be worth it. If you have lots of courage but not lots of money then go for it.
http://www.newegg.com/Product/Prod [...] 6814130402
If you get this one, you don't need to add a second card later. It wouldn't scale so well anyway, with 4 GPUs.

Or you can try this eVGA 9800GTX+ for $160.
http://www.newegg.com/Product/Prod [...] 6814130376

There's also the XFX GTX 260 for $270 with a free Call of Duty 4.
http://www.newegg.com/Product/Prod [...] 6814150310

This Corsair 750TX PSU is a huge bargain at $100 and can handle one 9800GX2 or two 9800GTX+ cards or two GTX 260 cards.
http://www.newegg.com/Product/Prod [...] &Tpk=750tx

The power goes something like this: GTX 260 SLI > 9800GTX+ SLI > 9800GX2 > GTX 260 > 9800GTX+