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The PC he was given was a shop bought little thing, the PSU when I looked at it before was pretty low power.. can't remember exactly.
One of the problem with the current available cards is that anything under a 7850 (256-bit) would have memory bus crippled down to 128-bit, which frame rate will pitfull if AA was applied due to the low memory bandwidth, not to mention most of them only has 1GB vram. You'd better off looking at 2nd hand old gen 256-bit cards. IMO something like a GTX560Ti 2GB/6950 2GB would be a good match with the i3, but trying to find one at the right price (under £70 ideally) might be a bit difficult if you don't have access to members market.
(Sidenote: 7850 2GB only worth around £90-£100 2nd hand at the moment...which is why I think it's simpler to just hand him your card than trying to find the GTX560Ti 2GB/6950 2GB for under £70)
Considering OP's kid's PC is one of those off-the-shelf PC using just a i3 and intergrated graphic, the chances are it is only of those generic 300W PSU or something, which would require upgrading anyway if anything higher than a 7750 was added to the system.
