Play Machines
Budget: <250GBPIf you can confirm that the PSU that comes with the case has a minimum of 18A on the 12V line then you may consider dropping the added PSU (which isnt that great anyways) and upgrading the CPU to the 3100 (the 3000 gets a clock speed bonus at the cost of half the cache). If not, this is still a fairly competitive system especially so for its price.
Budget: <300GBPBy sheer luck I was able to locate a X600XT (the alternative was to settle for a X300/6200 class card which is significantly behind in performance). In addition to the videocard change we've changed from a S754 Sempron above to a S939 Athlon64
Budget: <350GBPHere we spent the 50GBP to upgrade the videocard and CPU as well as switch to branded (as well as tried-and-true) memory.
Budget: <400GBPDid a lot with the 50GBP difference between this and the previous stepping: a phenomenal upgrade in videocard performance as well as a bump in the CPU
Budget: <450GBPDid some minor changes here and there as well as bump the harddrive up to a respectable 250GB.
Budget: <500GBPNot quite enough money to make the jump to dual core so we make motherboard and CPU change.
Budget: <550GBPSome shuffling around of parts and we've got a dualcore gaming rig.
Budget: <600GBPSome more shuffling: traded one top notch motherboard for another as well as got a first rate PSU
Budget: <650GBPA bit of shuffling and we can put a 7800GT into this box