He shouldn't need a new pc - Id try to hold off getting a new one if you can until the new windows comes out next year . Add 512Mbs RAM & either a new GPU or CPU or both should do fine. What sort of performance are you looking for?
You probably won't have an LGA socket on your celeron PC which the CPU asda_price recomends requires. Its more likely to be a socket 478 processor with a limitatation of either 400FSB (P4a), 533FSB (P4b) or an 800FSB (P4c/e). Unless its the latter I wouldn't bother upgrading your CPU.
You may mot be able to upgrade your Graphics card if your mobo has the 5700 integrated with no AGP slot. I think this is less likely with that sort of card. Another restricting factor may be the PSU supplied by the manufacturer - some use different sized PSU's for some BS reason.
Things you need to find out:
Mobo specs rather then Mobo -
Socket type (socket 478/LGA775) -
FSB (400/533/800) -
Graphics bus (i-AGP/AGP 2x/4x/8x/PCI-Express) -
Ram -
Speed (PC2100/2700/3200) -
then I would say to get the ultra-cheap Dell Dimension 8400
Never buy a cheap home (non-corporation) dell desktop if you can avoid it!
Your current video card is quite poor as it is the 'LE'
It is however still an FX5700 - not the worst you can get. Perhaps a GeForce 6600 might be ok - I havent looked at Gcards for a while now.
Birdman.