I'll make this much easier for you, don't go SLI and don't make preparations for going SLI later.
1) Much more expensive motherboard
2) Much more expensive power supply
Also, it's a waste of money just looking at the cards themselves. It's better to upgrade your video card every 2 years than to go SLI to future proof your rig. Consider this: Less than 2 years ago, I bought a 7900GT for $270. A second one would have cost me another $270 (obviously). My single 7900GT played all my games for the 1.5-2 years I owned it just fine. So why SLI 2 of them? To future proof my system to last longer. However a single 8800GTS (G92) 512MB would pwn two 7900GT's in SLI. And the 8800GTS (G92) 512MB costs less than $270 now. So take the money you save by going with a single card solution, save it, and upgrade your parts in about 2 years. It's almost always the best choice. Plus you can sell the 2 year old card reducing costs even more! After RMA-ing my 7900GT, selling it, and buying this 9600GT which doubled my 3DMark06 score, I only spent a net total of $30!
So get a solid P35 board, CPU of your choice, a single 8800GT (which will stay relevant for 1.5 years or longer), a 500W PSU, 2x2GB of DDR2, and 64-bit Vista. It will shock you how inexpensive the system is and the performance you'll get. Of course you also need a case, hard drive, CPU cooler and DVD drive.
EDIT: lol AARDVARCUS clicked his submit button a few seconds quicker than me, +rep for agreeing