I presume your main usage is gaming. For the games you will play and the resolutions you will play at, go to tom's vga charts to see if a good single card or a sli configuration will be better. Then pick. I am not a fan of the complexity of sli.
SLI has been a poor upgrade path in the past. It should be used only by
those who will not currently be satisfied by the fastest available single vga card
which is currently the 9800GX2. The new 9800GTX or 8800GTS-512 offer very good performance for the money.
To get SLI. you have to spend more up front for a SLI capable mobo, and a
more powerful SLI capable PSU. Upgrading a single card later with a
second equal card does not get you 2x increase, it is more like 1.1x to 1.5x depending on the game.
At that time, you will still be paying top dollar for a card that is closer to
being obsolete. A higher end card will likely appear before summer. It would be better to sell the old card and use the proceeds
towards a better new generation single card.
On your build, a 9800GTX would save you money. A non-sli mobo could save you $100, a smaller psu would save you $40, A E8400 would be better for gaming and save you $20.... etc... Pretty soon, even the 9800GX2 would not cost you more. My strategy would be to get An EVGA 9800GTX. If you love it, then great. If you still want more, then you can use the "step-up" program for the new 260 unit.
---good luck----