Mal made an excellent post, probably the most useful original topic related post in the thread, kudos Mal, well done...
a few more variables should be factored in before deciding...
one variable that hasn't been introduced into the discussion is the OP's psu, can it handle the extra power required to rub a 9800GTX...? does the card that he's thinking of buying use just one PCI-E connector (all the cards listed except one) or two (the 9800GTX)...? also, is the PCI-Ex16 slot next to a vacant slot...? I know at least one of the cards takes up two slots because of the cooler (the ASUS 9600GT)...
IMO, the best card out of the bunch is the 4850, it costs about $30 more once rebates are factored in, but as Mal mentioned, it is more future proof and will probably see even better performance once the drivers are tweaked a bit, if he wants Nvidia no matter what, the best choice is probably the Zotac 512MB 8800GTS for $125 after MIR that I've been posting about, it beats all three of the cards in the original post in performance and even though the price is back up to $149+SH after rebate, I bet it drops to $119+SH again by Thursday or Friday, they have 60+ units left in stock and there's no way they sell all of them unless they keep dropping the price intermittently...