Would be to just go with one card. There are new cards coming out every 6 months it seems and so do you want to pay the really big bucks to get a pair each time or would you be OK going back and forth over time?
My desktop has 1 7900 GTX with 512 MB of RAM that works great for me.
I don't think it is that hard to add a second card, though you do have to remember that there has to be room for the card on the board and have it fit in the case so that if you take a smaller case it may not necessarily hold 2 big video cards. Besides some cards are almost like 2 in 1 if you look at the 7950 GX2.
Link showing how some SLI and CF configurations compare. So for example they note a 7600 GS(256 MB) SLI gets 8.0 FPS while a 7900 GS(512 MB) gets 9.0 which is better for that game.
What is everyone's opinion on the advantages of an SLI or Crossfire setup? Would I be better off building my new computer with say 2 7600 256 cards or getting 1 7900 card with maybe 512mb? Also how hard is it to add a 2nd card later if I were to get a more powerful single card installed from the factory and then add another in 6 months when it is cheaper?