First, unless you are running a 64bit operating system then stick with either 3gig of ram or 2 matched sticks of 2 gig. A 32 bit system can only address 3 gig at a time.
Second, for tracking purposes you probably shouldn't run any plugins at all. That saves a lot of horse power. Use your plugins during editing.
Add on DSP cards are perhaps another option for you as far as adding audio engine power. That might be more bang for the buck than CPU/ram upgrade. Also, stick with Pro Tools 7.4 as 8 sucks down more resources seemingly. Digi will work the kinks (or maybe the Ramones) out of PT 8 eventually.