Assuming exchange rates and comparable purchase power, I would say your best $450 (£250) upgrade, with the most FPS for the buck, would be.
EDIT: REMOVED SEE BELOW (6600GT $208 (£115) (Hard to find but performs about 15% less than a 6800))
ATI RADEON 9800 Pro (128 MB) $200 (£111)
Athlon 64 3000 $145 (£80)
A64 Cooler $20 (£11)
<A HREF="
http://www.amdzone.com/modules.php?op=modload&name=Sect..." target="_new">DFI K8M800-MLV</A> $84 (£47) (Cheap mATX with IEEE/SATARAID and crappy integrated graphics. NF3 boards can be found for 15% more)
Old Ram (loose 10-20% CPU performance and may not work (but that’s a very small possibility))
$449 (£249) but it will probably end up costing much more across the pond.
You need to have a good Power supply with a 4pin ATX power connector or if you think your PSU is powerful enough 350w+, a $10 adaptor may do.
Dichromatic for your viewing plesure...<P ID="edit"><FONT SIZE=-1><EM>Edited by Schmide on 10/17/04 10:04 PM.</EM></FONT></P>