yeah, I agree, save some money, if its an issue, & do a SLI setup with 2 320MB GTS's, as carl said, assuming your PSU is up to it. (its my eventual plans on my setup down the road here, already run one, but leaving my system open for running two eventually)
one review I remember reading not to far back, 2x 320MB GTS's in SLI, barely nudged out ahead of a single GTX... so you should get close to the same as one of those... but then again, it all comes down to the games, as some don't run too well with SLI, others do...