Which PC for £500

Indeed. Always 64bit unless you're on a netbook or something.
The key thing with hard drives is only to have to access the buggers once, which means no memory pressure, ever. I find 2x7200rpm SATAs in a mirror is pretty good. Doubled read speed and losing a drive doesn't involve reinstalling the machine. Most Linux distros will do the setup for this in the installer now, too.

It depends what you're doing. Some things require read/write hdd access often and the ram you have doesn't help (Visual Studio is always doing compilation and shit in the background, writing to the drive). So, once you've maxed the RAM for your OS, get a fast hdd. Mirroring doubles your drive cost. I'd rather a fast drive with good backup routine but I can see the advantage of mirroring for uptime. I'll soon have VB's old desktop and at some point will probably move to SSD for the speed but we shall see.. mmm.. speed..