Heh! LOL@ Whatsit!
Hmm, my PC needs upgrading: More RAM, ok, that's cheap and easy enough. Oh, these new softwares need a faster processor to run, hmm, fiddly. And now, the graphics card can't handle it...
So, you spend, say, Â£500 on a desktop. How many people will actually be constantly swapping bits over? 90% just want it to work, and be reliable. The vast majority of PCs, like Macs, get little more than extra RAM added, and maybe a bigger HD. And as for 'oh, they're easy to upgrade', and "There's nothing as entertaining as swapping out hardware when you need some more power", the number of dead PCs I've seen in people's houses...
'What happened to your PC?'
'Oh, I tried to upgrade something, but it just went dead'.
Not everyone is a geek.
And the thing is, Macs don't need 'upgrading' as often. I have two old CRT iMacs, one 8 years old, the other, possibly 10. Both run OSX 10.4 without fuss. Granted, 10.5 is beyond them, but show me a PC older than about 3 years, that can run Vista without some serious tinkering.
My Mac Pro can take all sorts of 'upgrades'; HDs, graphics cards, PCI cards, SATA devices, Blu-Ray drives, processors, etc. Oh, and it can take up to 32GB of RAM. I think I'm sorted for a couple of years, yet!
Getting back to the OP; I suggested the Mac Mini, as it is such an easy solution. Quiet, very efficient, tiny, and packed with loads of good stuff for a little home media centre. Probably suits the needs of the OP, perfectly. And won't cost too much, either.
superlurch; you're not in London, are you? I'd have that G4 off you!