I did the specs from a 670 4gb to a 680 2gb and the difference was about 5-8% better, and to the 690 was about 10%, Couldnt justify spending almost 200-600 more for just a maybe 10% upgrade.
Plus i figure with the way i built it and my wattage for my psu i should be well within my overhead if i wanted to upgrade later, i think with everything in my machine i should have about 200 extra watts.
At the same time next gen card radeon 8000 and gtx 700 series *MAY* to use 4 gig of vram and 384 bits(nvidia)
If this wont mind u, its fine, u will probably need to upgrade this card on the 800 or 900 series, if nvidia want to name their card this way,
About ur psu, i dont really understand table top?
Logically, the heat from bottom mounted psu usually come from the case itself, case cant provide enough or adequate air to the psu,
Secondly, i suggest not to put ur case to near to a wall, i mean the back side.