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If he currently has a 8600GTS, then he'll need to sell that and use the money plus his current budget to get something like a 3870 or a 8800GT (provided he can get a nice little bit for it).
It's not worth 'upgrading' to a 3850. Might also be worth checking a well known auction site for prices on the 8800GTS 320mb/640mb and coming back here to buy it, of course.
Yes PCI-E 16x,
256MB isn't really futureproofing, or a worthwhile upgrade from his current 8600GT 256MB