Been 2 yrs since I built my last pc, the technology moves forward so quickly! Im sure others more knowledgable than me will be along, but at a glance paying £20 extra for a quad is good value, plus you will get 1333 mhz FSB which your mobo supports:
Rather than leaving 2 empty RAM slots using 4x 1GB modules will give you dual channeling for an extra £12:
Hard drive-wise, getting muliple drives in Raid0 with a fairly large block-size (with adequate backup) speeds up loading times significantly, so might be worth looking at a couple of 250gb drives with a 500GB for back up, you can still get loads of games on 500gb.
For the sake of £7 a good old floppy-drive is a must! (maybe thats me being old fashioned):
Its great fun building a gaming rig from scratch, even more fun extracting the best from it, go for it!!