SLI really isnt that nessassary for normal gaming, as SLI alone dosent increase frame rates.

huh?!? what!?!?

yeah i said it. the two video cards you just droped dime on may process light speed but if your CPU cant get the info there and the memory dosent have the headroom for the information then the GPU sits there idle waiting for information to render.
in high end gaming, yes it makes a difference.
example: the Intel Quad core extreme ($1400 @ 3.2GHz) can litterly spray information out to be processed and having 3 Video cards SLI'ed takes full use of this information.
so if you have 5-8 grand to plop down then sweet!
SLI is best if you can afford to get BOTH cards right off the bat.

i've had 3 systems capable of SLI and have yet to buy a second video card. why? because i bought the best card at the time and had the intention of getting another one, however things come up and the next time i got to it the next gen card was out and it was a waste to get a second Ultra card of the previous cycle.

if you get the system above it will be plenty fast and capable of at least 24 months of cycles before you may want to upgrade. and at around $270 for the system your looking at about $10 a month.

and who knows AMD may get there act together and at christmas you get to drop a 3.5GHz Phenom FX into your machine, or a brand new Nvidia 9800 GTX Ultra and your only out ~$300 now

ALWAYS consider upgrade potential when settling on gear