Dillinja - it's safe to say that in a forum like this where we mostly build our own machines, you're likely to get a lot of recommendations to do the same and build it yourself. While i would always build my own rig now, I only made that first jump into DIY when I had developed a lot of background knowledge and had a couple of friends with experience of self-building PCs - so i could esily get help.
We're always eager to help, but you might find that our online support is not fast enough when you're in the middle of a build. It sounds to me like you really do want to buy off the shelf for now (and maybe get into building later on) so as far as you spec goes, I would say that £600-£800 is reasonable.
For that kind of price though, you may end up with socket 754 rather than skt 939, and the case and PSU maybe a little cheap. Make sure you're not getting things like micro-ATX mobos with onboard sound and vid, rather than seperate video cards. If you want to play top end games, the vid in a basic off-the shelf PC may well be below what you need. A common weakness in shop-bought systems is the minimal consideration they give to things like internal cooling - use of only a single fan and so on. Also keep an eye on the warranty agreement that's included - what it costs for calls to their service helpdesk etc. What upgrades can you do yourself without voiding the warranty? etc...
Anyway, I hope the advice you got from here so far is useful...and come back if you want to get views on systems you think might fit your bill