I have a Dell XPS410 with a 7900GS, 256 mb video card. I'd like to add a second card and do an SLI setup. Does the second card have to match the first one, or could I use, for example, a 7900GTX 512 mb card?
Would both cards revert to the lower performance of the lesser card, or what?
Your next card will have to be a 7900GS. The good news is that any 7900GS will work. It used to be that the second card had to match exactly: part number, BIOS version, and brand name. Those restrictions were lifted with recent NVidia drivers.
Keep in mind, you'll also need an SLI bridge... not sure if you got one from Dell when you ordered...
I still don't think SLI is the best option for an upgrade path (unless you get the second card for dirt cheap or free). At this point, you could buy a new 7900GS for around $155 on newegg. OR you could buy an 8800GTS 320MB for $300 and sell your current 7900GS on eBay for around $120. For $180 (vs $155) you get a single card solution that whips the pants off two lesser cards in SLI.
Forget About SLI.
Get an ATI X1950Pro/XT or 7900GTX or 8800GTS 320/GTS 640 :twisted: /GTX instead.
But remember to check your power supply and see if your power supply can handle newer graphic cards :!: