Okay, if you've read my other two posts in this topic, you'll know that I'm hellbent on converting you to PCI-E-ism.
I'm assuming you still have 150 British Pounds, correct? According to XE.com, that's $295, so those're the bounds I'll use.
($120 after rebate)
Total -- $299 - $20(rebate) = $279 = ~143 Pounds.
Okay, So I got you a nice video card, a good motherboard, and an okay processor. The motherboard should do you good for a while, so you won't be upgrading that.
The Processor is okay, but I'd upgrade that to a Core 2 Duo first chance you get.
The Video card I'd upgrade after the processor, it should play any game at medium to high settings with okay fps, according to the fine people here at Tech-Forums.net