Well, a local computer shop where I live does a complete PC with Win 7 pre installed for £275. It's a 2.6 dual core with 4gb ram, a dvd burner and a 250gig sata hdd. I recon that is good value for money and plan on getting one for myself in a week or 2.
It has a Gigabyte mobo which is a decent enough make.
It has on-board graphics but also has a pci-e x16 slot for a good graphics card if needed.
I have so far gotten 3 of those machines for a client and she is very happy with them and tells me they are quick, quiet, and reliable.
For those that are considering building your own PC...keep in mind that the computer can only run as fast as its slowest components. Therefore, pay close attention to your secondary storage (hard drives). This is where you want to spend a little bit more to get the best performing hardware.