That's a hard one, you could go for a better video card. But I think I'd also consider a better power supply if you go that route.
Here's something else you could consider if you want to reuse the memory you have.
http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.293300
Combo for 163 gets you a new Athlon 2 quad core processor with an asus mobo.
Or here is a 2.9 ghz athlon 2 triple core with ecs motherboard(granted not a big ecs fan myself, but the price is good, and it's 125 for both after rebate. About 140 before.
http://www.newegg.com/Product/ComboDealDetails.aspx?ItemList=Combo.289560
With your budget, I'd be thinking about maybe the triple core.
So 125 there.
Was looking at a 4850, but saw a gts 250 for 100 after rebate, 115 on sale.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814162034
Some reviews said they've had problems with the card, I don't have that card, I've actually got a 9600 gt from galaxy and have had no problems on the 9600.
So 225 so far
If you go with these upgrades, consider a new power supply.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817139008
Puts you a little overbudget and don't know the specs of your huntkey, it's your call whether or not to take a chance on it. But if you go with these parts, reuse your ram, case, optical drives and hard drives, it may be a good upgrade. Should definitely see some improvement on the video performance.