i don't think your processor can handle a 3xxx series, you'll bottleneck it and waste your money.
At the most, I'd get a x1900/x1950 if you can find out, or an HD 2600. chances are your power supply can't support any of these cards anyway.
Now, that's if you have an AGP slot. If you have a PCI-e slot, then it's a bit different.
i'm not a pci-e expert, seeing that as i still run agp.
However, if you have a pci-e slot, then I would still go with the same class card, maybe a 3850, but only if you can afford it easily and you have a PSU powerful enough to back it up.
Chances are it will bottleneck, but the 3850 will allow you to keep it for future upgrades. If in a few months you decide you want to build a computer, thats one less thing you have to buy. Honestly thinking about your current situation, even though a 3850 may bottleneck it just seems to be the best for your money. This is of course without upgrading your whole system.
1. If all this fellow wants to play is World of Warcraft, all he needs is a half-decent card.
WoW is extremely easy on the videocard.
A sub-$100 2600 PRO would be more than he'd ever need for that game, even at 1650x1050, and it would show him a COLOSSAL increase over his integrated video.
2. If he DOES want something more powerful for other more demanding games he might want to play, I sure as hell wouldn't jump him to a 3850 on a single core Pentium 2.8 GHz. A Radeon X1950 PRO would be much more suitable for him, and he'd need a new power supply for that, even.
Sure... maybe if he plans to keep it through his next upgrade a 3850 would be a good investment, but does this guy even play games other than WoW? And does he plan to upgrade the rest of his syustem in the near future? We're assuming a bit much methinks.