ok, most important and often overlooked - motherboard, dont skimp, and PLEASE stay away from pchips, ECS, jetway and the likes, Asus, Abit, Intel, Gigabyte all decent, as are Asrocks although performance wise they arent too good. You usually get what you pay for, looking at £50 up
CPU - depends, very confusing to boot, probably looking at something like a 600 series intel or athlon 64 single core about 3500 ish round about £60 mark
Case + psu - £25 up
Graphics card - totally up to you, onboard up to £1000's
HDD - 250 gig about £60 ish
RAM - again confusing - RAM isnt RAM ! it has different speeds (as well as different types) and speed makes a vast difference. Theres generic stuff which wil probably suit you fine, reckon about £40-£60/512 DDR or DDR2 - do yourself a favour and dont go for the slow stuff.
dvdrw about £30
Im tired and the prices are off the top of my head but shuold give you at least a bit to go on, you are probably now thinking "hmm, cheaper to goto pcworld or dell, they sell high spec machines for £499 inc 19"tft"
They sell machines for that, but not high spec, celerons, pentium D's, Semprons, stay the heck away from them if you want anything other than an office pc.
Possibly the best option if poss is to grab the necessities and uses old parts (if you have any) then expand later, for eg use your old hard drive, might be small and slow but gives you £60 more for your system, get a new drive next month.