Why are you thinking of upgrading the graphics please?
I reckon your old tech single-core CPU will be quite a considerable bottleneck to the 9600/8800 cards, & will require a more powerful PSU (= more expense).
If your PC has a Dell 350w PSU or better (open case & check this), your best bet may be a 7900GS click here or 7800GTX click here (both £60).
Next best could be a HD 3650 click here (£50, add something for £1 to get free delivery on your order). This is a little better than a 8600GT, but not as good in games as the former two suggested. To get some idea - a 7800GTX & 8600GT compared click here.
Not worth spending more than £60 to upgrade the graphics. G