I'd probably go for something like this paired with an ASRock Dual-VSTA motherboard however I suspect you will have to source the motherboard (~£40) elsewhere as OcUK don't seem to sell it for some reason. The motherboard allows you to use your current AGP graphics card for the moment and you can change to a PCI-E graphics card with the minimum of fuss.
NEC AD5170 18x18 DVD±RW Dual Layer ReWriter (Black) - OEM £16.99
(£19.96) £16.99
(£19.96)
Intel Core 2 DUO E6300 "LGA775 Allendale" 1.86GHz (1066FSB) - OEM £94.99
(£111.61) £94.99
(£111.61)
Arctic Cooling Freezer 7 Pro CPU Cooler (Socket 775) £13.99
(£16.44) £13.99
(£16.44)
Samsung SpinPoint P SP2504C 250GB SATA-II 8MB Cache - OEM £44.99
(£52.86) £44.99
(£52.86)
Antec NSK6500 Super Midi Tower Case - 430W PSU (CA £58.99
(£69.31) £58.99
(£69.31)
GeIL 2GB (2x1GB) PC5300 667MHz Value DDR2 Dual Channel Kit (GX22GB5300DC) £129.99
(£152.74) £129.99
(£152.74)
Sub Total : £359.94
Shipping cost assumes delivery to UK Mainland with:
City Link Parcel Next Day (Delivered Mon-Fri)
(This can be changed during checkout) Shipping : £9.95
Vat : £64.73
Total : £434.62