Not really a good choice as the motherboard has onboard graphices which indicates it's a cheapie.
Also, the case and power supply (PSU) aren't listed.
What size monitor do you want? For gaming 20" is the minimum, 22" preferable, which will cost you from £130.00 to £200.00. Let's say you buy a good 20" for £150.00, this leaves you a budget of £650.00 for the computer.
Do you need keyboard, mouse & speakers? If so, we're looking at a budget of £600.00 for the tower.
You could build yourself a decent gaming machine for that money, nothing spectacular but certainly adequate, but buying ready made you'll probably have to compromise.
Look for motherboards by Asus, Gigabyte or DFI without onboard grafix, 2Gb of memory will do for now, at least a 500W name PSU and go for an AMD based system as they're cheaper, a dual core CPU will be fine, AMD quad cores aren't actually all that good.
I'm not up on the latest range of ATI grafix cards but an Nvidia 8800GTS will do you ok within your budget.
XP or Vista? Tricky, as Windows Seven is supposed to be out in 4 months' time which will possibly outperform Vista by a large margin.
I certainly wouldn't buy Vista now as it will be redundant shortly.
If you can still get hold of a copy of XP for less than a hundred quid I'd go for that then upgrade in August to Win 7 if you feel like it.
XP, although outdated now, is actually faster than Vista and is a mature stable operating system after being around 8 years in existence. And XP will run a lot of older games that Vista won't.
If the OS, monitor, keyboard, mouse, speakers are included in your £800.00 budget, this leaves you 500 quid for the tower unit.
You'll likely have to skimp a little on the grafix card but you're still in the running.
