Look at min frame rate. SLI does a great job here, and it is what you "feel" when you play a game. Also, two 256 MEG cards in SLI perform more like a single 512 MEG card since each card is doing about half the work. Not 100% but close. Run Doom3 with a single 7800GT card but in SLI with two, you'll never go back to one. 30% "maximum frame rate" isn't what matters, 30FPS minimum @ 1200 x 1600 instead of 24FPS is. Be careful what is important to performance, and what performance actually is to you.
-slower CPU (3800+ X2).
- standard PC3200 RAM.
-7500RPM hard drive.
-ASRock SLI motherboard.
- 2 x 7800GT.
equals greater performance in real game play over;
-fast CPU (4400+).
-PC3500 or faster RAM.
-10,000 RPM hard drive.
-A8N32 SLI motherboard (or pick your bran high-end).
- 1 x 7800 "ultra" single card.
To make the above "best" work, expect to spend big buck with another video card in order to make slightly higher frame rates. It will be faster, and hugely more expensive, too.
That said, I do think that a more current single or dual card solution will be better for longevity and ownership cost per time owned. Get a 7800GT or wait till the 7900 series is out and buy the card that seems to be the best price point for your needs. It may still be the 7800GT or 6600 series with a price drop.