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Fortunately for us all, 300D, in addition to being a troll, is also wrong. :)

Luckily the ignore list is there for a reason.

OP, yes you can install and run Tiger and upwards on PC systems. Even the developer release of Lion has been made to run on a PC. However, it's not for the faint of heart and is not advisable if you're not willing to learn, read a fair bit and deal with having to be a little careful when coming to system updates.

Since you're in the UK, the legality side of things is irrelevant since the EULA is not legally binding at all in Europe.

All in all though, it sounds like you would be better suited for a second hand Mac Pro (or a SB based iMac when they come out). If going down the used Mac Pro route then the 2008 series octos are the best bang-for-buck since not only are the eight core, they also support EFI64 which means that you're unlikely to be locked out of any graphics card upgrades any time soon.
 
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