I wouldnt know to be honest, but your best off not getting one built by a company since it limits the upgrading. That is why the RAM is soo slow, since its probably a cheap motherboard which wouldn't be too good with a new processor anyway, i.e the Pent 4.
If I had to limit a guess, I'd probably say about £200 (about $235), though again, I'm not too sure. I'm just thinking you see of how much it costs to buy a new system, and they come rather cheap these days.
actually kage has a point, looking at the comp more closely i would say 200-300 because of the slow memory and terrible cpu, as for your a new system tell us a budget and we'll list what u should get
a 500-700 system would be quite decent and if u pick the right parts u can overclock it and have a pretty quick comp, as for the hard drive i would just sell it with the comp because its probably IDE and you're deffinately going to want an SATA drive with your new comp