When you have such a old machine large returns can be made from fairly small financial outlays. Id imagine £50-60 would treble or quadruple framerates in most games from what he has.
Personally, I'd try to get a QX6700, which are probably selling for peanuts ATM. The E6600 will bottleneck any big GPU you'll throw at it. (Was running my 5870 at 80% with a Q6600@3.6ghz. Imagine a E6600 now : \)
If it was me I would but something like a Nvidia GT730 or 740, low power no noise!! I recently built a dual core machine for my lad using the G3258 and a GT730 and its playing simple games like the batman and Marvel Lego series @1080p, medium settings and 50>60fps.
