OK, try these on for size.
CYA clause
Bear in mind that you are getting these suggestions based on my likes/dislikes - you may have different preferences.
Prices vary almost day by day let alone weekly & new products are coming onto the market all the time, as I said it's a moving target.
Even going for 2HDDs, 2 ODDs, the dearest case, gfx card etc. should still come in under £1200 inc. VAT for the components. If you really want to spend £1500-2000 I can do it for you though ...
Going for the cheapest options, single drives etc. is probably ~£700.
Case
Akasa Zen £29
Coolermaster RC-330 £26
All sorts of options £80
PSU
Corsair HX620 £88
FSP Epsilon 600W £82
Motherboard
abit IP35-E £65
Gigabyte P35-DS3R £70
abit IP35 £80
Gigabyte P35-DS3P £87
abit IP35 Pro £120
CPU
E6750 2.66GHz Dual £120
E6850 3.0GHz Dual £170
Q6600 2.4GHz Quad £170
RAM
2GB DDR2 6400 4-4-4 £65
4GB DDR2 6400 5-5-5 £180 (2x2GB not 4x 1Gb)
2GB DDR2 8500 5-5-5 £110
HDD
HHD1 HGST 320Gb 16Mb £45
HDD2 (if requd)HGST 320Gb 16Mb £45
ODD
DVD-R/RW1 Samsung SH-S203 £19
DVD-R/RW2 Samsung SH-S203 £19
Nero/PowerDVD oem £8
Floppy
Sony £4
Mitsumi FA404 £15 combined FDD/card reader
Gfx card
320Mb 8800GTS £190
640Mb 8800GTS £225
512Mb HD2900XT £240
Audio Try onboard first
OS Win XP SP2 OEM £53
Vista Home Prem OEM £68
Modem Get a router/4-port switch? £30