With a 4.3GHz P4/HT, CPU loading should not be much of a problem at all unless you really lay the real-time and simultaneous FX on thick on many, many tracks. Memory size can be a bottleneck, but I'd suspect with that processor that you probably have at least 1 or 2 GB of high-speed RAM, so that wouldn't be an issue either. If you have 1Gb or less, then you might consider investing in a doubling of your RAM capacity to help out a bit.
Boi is right, the main CPU loading comes in when you are using stuff like impulse/convolution reverbs, guitar/amp modeling, heavy calculation stuff like that. EQs, compressors, filters and the like shuld be to your CPU what a feather is to a linebacker.
If after all is said and done you continue to have problems, make sure you unload as many TSRs (e.g. firewalls, virus checkers, etc.) from your CPU task list as you can before you start your session. This of course means physically disconnecting from the Internet while you're recording and mixing, but that shouldn't be an issue. Multitasking while mixing usually just results in bad mixing anyway
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And finally, but perhaps most importantly, Look into the technique of "locking" and "unlocking" tracks during mixing. Locking a track (usually just a single mouse or keyboard click) simply means telling your software to render to disc a temporary WAV copy of your track in it's current state, with all automation and effects intact. Doing this means that when that track is played back, there is only a pristine wave file to play and no effects need to be calculated in real time. If you want to go back and change that track at any time, you simply "unlock" it and the software will revert back to using the original untouched WAV file with the real-time effects still overlayed but un-rendered.
If you're working on 8 or 80 tracks in your session, you can simply lock the tracks you are not actively working on at the moment. This will preserve their FX and automation information (and play them back properly) without requiring the CPU cycles for those effects. Keep the track or tracks you're working on unlocked, leaving all your CPU cycles just for those couple of tracks. When you're done with those, lock them and unlock the next track or tracks that you wish to tweak, and so on.
G.
