wow, what kinda polys are you pushing? I have a 6600 n' 4 gigs and haven't had too much problems with XSI + Zbrush I would say Intel's entry level quad cores would be a good starting point. i7's are brand new and pricey. I wouldn't really bother of looking into upgradeable mobo and cpu wise. Buy the time a better…
Also keep in mind that Zbrush doesn't use any 3D acceleration features of your graphics card, it's only software rendered. So if you want to work in high resolution, get an even faster CPU. Dual core is a must, quad I'm not that sure about - but if you want to do some nice GI renders as well, then go for it...
lol what stuck? I have the same system and find more often then not the programs i use can't take advantage of my 4 cores, and are barely pushing my system than it is being maxed out. Would like to know what crazy sort of stuff you're doing, as my q6600, 8gigs and 8800 gt can handle just about anything! Sure i would love a…