The size of the PSU is going to be the decisive factor how high you can go in a card. The GTX460 will be a nice match with your current setup (pending PSU size) and a big upgrade from the 9500GT. If you want to save some money you could be looking at GTS450 as well.
Thanks for your help rolli59, not sure what my size my PSU is from memory - probably nothing fancy. Think I'll probably go with the GTX460 the price isn't that much more than the other type. Its always nice to be reassured when you're about to part with money! ;-)
As usual, Rolli59 gives good advice, just be sure the PSU can handle a GTX460 before parting with those hard earned drinking vouchers.
Oh, and I notice you have a micro ATX motherboard, if the case is also fairly small, check the card will fit, most will, but Murphys Law is always waiting in the wings.