I can do £488.61 Delivered.
At this price your gonna have to make sacrifices in certain areas, Forget fancy soundcards and speakers (Monitor has built in Speakers) & Mobo has 7.1 Audio so more than sufficient, besides if you cant get on with the speakers plug in your stereo speakers.
A Radeon 4850 would be nice but a Radeon 3850 @ £58.74 is a bargain, I am running the AGP version from Sapphire and im maxing out Cod 4 with no drop in frame rates, lags on Crysis but most things do, Will go much faster in a PCI Express X16 Slot & not suffer the bottleneck of an AGP X8 Slot and Single Core Cpu.
Motherboard offers an excellent upgrade path for Gaming with CrossfireX support for upto 2 Cards.
PSU should be enough although if you want to run dual cards its advisable to go out and get a better Power Supply
2.7ghz Dual Core Processor is fine for gaming and there is room to push it upto 3.0ghz on air but will need something better than the stock cooling.
Cheapo Case but with Power Supply Included Bargain!!
160gb Hard Drive should be enough for Games too.
All in all not bad if you ask me.
Vista 32 or 64 its upto you, 64 offers you the chance to get upto 8gb of Ram as another upgrade for the future.