I just bought a new system that I'm going to attempted to build.
Western Digital Caviar SE WD1600AAJS 160GB 7200 RPM SATA
1 AMD Athlon 64 X2 6000+ Windsor 3.0GHz Socket AM2
1 Rosewill R5601-BK 0.8mm SECC Screw-less Dual 120mm Fans ATX Mid Tower Computer Case - Retail
1 EVGA 320-P2-N811-AR GeForce 8800GTS 320MB GDDR3 PCI Express x16 HDCP Video Card - Retail
1 ASUS M2N-SLI Deluxe Socket AM2 NVIDIA nForce 570 SLI MCP
1 CORSAIR XMS2 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 (PC2 6400) Dual Channel Kit Desktop Memory Model
1 ORION Pro Series 18" 180 and 90-Degree-Angled, SATA (SERIAL ATA 150) Cable, 2-Head Model 700-201-05 - Retail
Item #: N82E16812104557
Return Policy: Standard Return Policy $3.49
1 Rosewill RP550-2 ATX 2.01 550W Power Supply - Retail
Item #: N82E16817182017
My question is this. say if I go sli add another card down the road, does it have to be the same card? Could I get a better one like maybe the 8800 gts that has more ram or could I even buy an ati product would any of this work. I tried doing a search and nothing really came up. so sorry if this question has been asked before. thanks alot
For basic information go here: NV SLi Zone
watch the videos, specifically the one titled "Pairing graphics cards for SLI"
But to directly answer your questions: Yes you can mix GTS' with different amounts of memory and clock speeds. The only thing that matters is that they are both 8800 GTS. SLi refers to linking two Nvidia graphics cards. Crossfire is the Ati equivalent. A Nvidia card CANNOT be paired with an ATi card to form SLi or Xfire.
If you're just using the extra card for multi-monitor then it doesn't matter if it's an ATi or nV. I would recommend the same generation for driver compatibility but if you use the other card for just extended desktops etc, then it will work under the windows multi-monitor management.
You can add a better GTS with more memory later for SLi, but it wouldn't be directly supported you need a tweak, and it would also simply use only the amount of memory on your current card, so therefore a 640MB card would offer nothing in SLi mode and act like a 320MB card.