I would build your own, not buy from Coment, £700 is a good budget to be working from..
Easily get a 64bit 939 processor (3200+ or 3500+) and a gig of Dual Channel RAM,
A graphics card,
160GB HDD
You could go for this, but you'd have to spend another £200 to get it running, minimum!!
http://www.novatech.co.uk/novatech/s...e.html?BB-X2DS
But seriously, if you haven't built before, maybe try a barebones bundle, you still get to choose what size HDD, and what graphics card to have, but the hard work has been done for you... Take alook at this:
http://www.novatech.co.uk/novatech/s....html?BB-NF435
A 3500+ 64bit Processor, 512MB of Ram (upgrade this in a year / 6 months maybe??)
£360
DVD Writer - £27
http://www.novatech.co.uk/novatech/s...tml?PHI-16DVDB
160GB SATA Hard disk - £60
http://www.novatech.co.uk/novatech/s....html?WD-160JD
Nvidia 6600GT PCI-EX Graphics Card - £112
http://www.novatech.co.uk/novatech/s...html?NOV-66GTE
Microsoft Wireless Keyboard and Mouse - £40 (I've bought these for under £30) so..
http://www.novatech.co.uk/novatech/s...tml?MS-S51-000
That's £599 for a pretty quick machine.. The only thing left is a monitor, but you never specified whether you wanted one or not, or if you wanted a TFT monitor or a CRT one... but you have £101 to spare
Don't go with the cheapest, it makes a BIG difference in computer systems, my PC is built out of good quality components, the cheapest thing it has or the only cheap thing is the PSU and you should never skimp on this, I am saving up for a good one ;-)
You can pick up a TFT for £135 but I wouldn't recommend it, save up for a decent one..
.
:-) I hope I've been of some help. if you have any questions, just make a post..
www.ebuyer.co.uk are usually pretty good for components..