Just thought I'd give an update...
I finally went with the following:
Intel Core 2 Quad 2.33Ghz
Gigabyte GA-EP45-UD3LR Intel P45 (Socket 775) DDR2
Gigabyte GV-N98TOC-1GI GeForce 9800GT ATX HDMI
Corsair XMS2 4GB (2x2GB) DDR2 PC2-8500C5 1066MHz
Seagate Barracuda 7200.12 500GB SATA-II 16MB Cache
AOpen ES45C Mid Tower ATX
Novatech 750W ATX "SILENT" Power Supply
Samsung S223C 22x DVD+/-RW 8x Dual Layer DVD-RAM SATA Black - OEM
My original idea was to build a low cost but capable 2nd computer to do all my internet stuff and stream media on to my television, while leaving my primary pc free (and uninterrupted) to do all my gaming stuff. That wasn't how it turned out though, as I have basically built an identical pc to the one I already had and it cost a lot more than I originally intended to spend (£520 - the one I already had cost £660 from a computer shop). Still, it does all I wanted it to do and more.
I've just finished building my Dad one as well. Pretty much the same, but he opted for an Intel Core 2 Duo 3.33Ghz. Although he likes playing Call of Duty, his main interest is graphics, and so spends a lot of time rendering (or more accurately, drinking tea while waiting for the render to finish). Above all other considerations, cutting down that waiting time was his main concern. When we researched it we concluded that he would be better spending his money on a faster Duo than a more multitasking Quad, and it has proved to be a good choice. He did spend twice as much on the processor than I did (£198), but cut that cost down by opting for the cheaper Duo instead.
Both computers are running great. Given that I'd never built a computer before I'm pleased with the end results, even if they're probably not the best components I could have bought. The fact that they work, and work well, is all I was really concerned about...
Thanks for all your help guys, much needed and much appreciated!