This is not just for doom3 but i have found some games i have are suffering with my 512mb ram like star wars galaxies everquest and istaria.
I have 2 x 256mb corsair xms 3200 cl2 chips in at the moment but they are not a matched pair. I do have a 512mb stick also corsair xms 3200 cl2 in a 2nd pc. I think getting a 2nd 512 chip would be better than running 4 x 256 (if thats possible) but would perfer a matched 1g pair so i can push the memory speeds higher as the non matched pair is stopping me at a fsb of 250 at cl2.
edit: just won a 1gig xms 3200c2 twinx pair on ebay so thats the ram sorted
Next is a external box for the water cooling and a 2nd loop for gpu and chipset this should allow me to run the fans lower on each loop than i do at the moment plus i dont have the chipset plumed in yet. I would realy like a full gpu cooler for ram and gpu but dont have the time to make one at the moment
current machine spec.
asus p4p800
p4 2.8C running at 3.5ghz fsb 250 at 1.52 volts
fx5900se 128mb overclocked to full fx5900 speed
soundblaster audigy
cascade cpu cooler
DD gpu cooler
hydor l30 pump
thermochill 120.2 rad
Hardocp have some benchmarks on doom3 here
http://www2.hardocp.com/article.html?art=NjQy