I would stick with your E6600 mate, Overclock it a bit to about 3Ghz (should be easy enough) and then upgrade to an x64 version of an os.. I recomend Vista (or wait for Win7) But at least xp x64 untill win7..
Youll find that multiple apps run better in x64 with more ram
You can apply to MS to get the X64 version of your OS for the postage price (about £5, not sure about euros) and use your existing key

I know saying "I would" sounds silly.. as I would ditch that system, start fresh with a huge budget and go all out with quads, 4 GPUs, 8GB Ram, Watercooling ect.. as thats what I did

But that doesnt make my recomendation any less valid
