But he owns a 320 already, so it'd be silly to go 2x640... he'd be paying almost $400 for a setup that is beaten by sli setups he could get for $250-350.
Honestly, with all the disadvantages of the old g80 gts' I'd avoid buying them like the plague. Even @ $110-120 (what Im seeing 320's at), you're close to a 9600gt or you can get an 8800gs @ that, which are comparable or better performance in most scenarios at much less heat and power draw.
Two 320s vs a single 512 gts would probably be give and take. I'd guess the 320's would average a little better... but at much higher power, much higher heat, much lower likely resale value, no chance of improvement (you could still SLI the 512 card later).
If ya can get a 320 for <$75 AND the power and heat are no problem for you, then I'd consider it... but I'd still probably go with the 512.