onehunga,
I would say upgrade now and wait till the new hardware comes down in price before upgrading again. (If you have someone coming to the U.S., have them pick it up and bring it to you.)
CPU: AMD AthlonXP 2500+ Barton Core
Motherboard: ASUS A7N8X-E nForce2 Ultra400 or the MSI K7N2 Delta-ILSR
RAM: 2 x 256Mb PC-3200 Micron/Crucial/Corsair (All the same chips.)
Video Card: ATI Radeon 9800Pro w/128Mb or the GeForceFX 5900XT-VTD128 w/128Mb
Keep your hard drive and optical drives, case, keyboard, mouse, monitor, and speakers.
If you could get it in the U.S. then:
2500+ $83.00
nForce2 Motherboard $117.00
PC-3200 $100.00
9800Pro $228.00
Total $528.00 or £277.89 (using Exchange Rate of 1.9)
Take Care,
Richard