I cant believe at the price of HDD these days, anyone who doesn't have Raid. The performance increase is huge and at relatively little cost.
You should buy ram at least PC4000, don't get PC3200 that will only just run yourmachine at stock. you need faster ram if you wish to overclock and still run your memory at a ratio of 1:1 (much faster)
And buy 2 DDR stick for dual channel.
Here's how the ram frequencies work:-
To make things easier, say you had a 3Ghz P4 800Fsb HT
Your machine would run at an Fsb of 200Mhz, This is quad pumped to make the 800Fsb.
Your CPU is Fsb 200Mhz x 15 to give your 3Ghz
Your memory should be double the Fsb (200Mhz), making 400Mhz Ram, Pc3200
If you then overclock by say 20Mhz on your Fsb. Your machine will run at 880Mhz (220x4 quad pumped).
Your CPU is fsb 220Mhz x 15 giving 3.3Ghz
And your memory would be double the fsb (220Mhz), meaning that your ram would be running at 440Mhz
Pc4000 is guaranteed to work at 500Mhz (an fsb of 250Mhz)
Simple isn't it?