What Should I buy?

I have got £150-£175 to spend and i was wondering what you guys think i should buy, my spec is as follows:

Abit NF7-S rev. 2.0
Athlon 2400+
512MB Dual Channel DDR400
Radeon 9800pro
430w Enermax Variable speed PS

I want to be able o run Half Life 2 and Doom 3 at a reasonable speed (i currently have doom 3 installed and it runs reasonably well well but i want MORE!)

Please give me your advice to what i should get, i have been considering a case but i have decided i want more speed!

 

more ram? and/or arctic cooler to oc your radeon 9800 pro

 

slap a fatter cpu in there, perhaps a 2800+ or a 3000+ and OC.

 

I suggest getting a barton and another 512 megs of RAM-that, or selling your old 512 megs and getting a 1 gig RAM kit from OCZ

 

Maybe a mobile chip for better ocing and definitly more ram. You cant have enough ram in a computer these days.

 

you see the biggest increase from more ram or a new gfx card.
The 9800 should be fine (if anything, overkill) so i'd surgest going to 1gig of ram.