Those systems would almost certainly be sufficiently outdated that you couldn't perform much of an upgrade. Getting a reasonable CPU would require a motherboard upgrade, which would in turn require a change of RAM, and by the time you've finished you've probably replaced every component in the PC.Dylan just buy a cheap PC from Here and then do a massive upgrade. Thats what im gonna do. Sure its all the way in west melbourne but its worth it.
CPU: Intel C2D e6400 $190
Mobo: Gigabyte DS3 $140
RAM: Corsair Dominator 2GB DDR2-800 $300
Video Card: evga 7950GT $215
HDD: Seagate 320GB 16mb $95
PSU: Corsair 520W $130
= $1070
I know it's a bit expensive without the case and stuff....
I'll do a revision a bit later, right now, I just made the list quickly off the top of my head.