Personally, what i think your looking for is a graphics card that is to meet your sons requirements at minimum as his ATI 7000 isn't cutting the job, an Nvidia 6200 or an ATI 9250 isn't going to make that much difference, they are better cards, but not much performance for your money over the ATI 7000, what i would do is get a solid mid-level card, a card that will be slightly more expensive, but is going to last alot longer for your sons requirements.
I would recommend (depending on your budget):
ATI X600 / X600 Pro
ATI X700 / X700 Pro
Nvidia 6600 128MB (£70-£80 - about $130-140
ATI X800 128MB 128bit (£85 - about $130-140)
I personally think these are your best bets, but i am assuming minimum he has AGP 8x & Socket A, if you list his motherboard / computer details we can help you out alot more.