Many thanks for all the advice on this. Shortly after I posted, my MB blew up & suddenly I was looking at spending more ££ than I intended.

So I decided to spend even more than I should & but a whole new base unit -
Intel Core Duo E6300, 7600 GT, & 1 gb RAM -& suddenly I'm playing a whole different game. It's very cool.

I even managed to put my own wireless card in it without mishap, & then it struck me quite how simple changing components really is (within reason of course). These PC repair shops are charging money for old rope.