a 4890 would be laughably wasted on an old single core cpu so why spend that much money to use that much power, noise and heat for nothing? a brand new $45 4650 with a good warranty is something that he can actually use though. plus when he does another build later he can just keep the 4650 for a back up card.
I can't resist a good deal, and honestly you have to admit you won't see this kind of performance at this price range for at least a year or more. Nvidia has released nothing new and the closest performing cards the GTX 275 and 5830 are $240. At $125 in a year I predict you could get a new 5770 or the latest refresh of the G92 (330?), both of which will be slower than the 4890.
4650 at $45 is an average deal, and I predict by black friday the card will be less than $20 after rebate. I've already seen 4550s priced at this level so I think the only variable that remains is whether or not there is stock remaining by then.
So as I see it. I could build a pretty nice system around the 4890 for say roughly $300 which could play practically anything, or I could waste $45 on a card that will take up space in my closet.
btw here is how I figure the price would look on the system. I'm near a Frys so maybe OP can't imitate this:
$125 GPU
$60 CPU + MOBO (athlon II X2 250)
$50 RAM
$50 PSU
Reeuse case, DVD, maybe spring for new HD.
Comes out to about $285.