I'm using that computer with an HD4 accel along with a Digi expansion chassis.
a.) I use everything TDM except for altiverb I use RTAS and can get 4 or 5 Stereo reverbs successfully I purchased the TDM version but apparently it isn't compatible with my expansion chassis.
b.) Toast 8 is killer for data, etc. But I still use Jam for burning masters, cd text, and printing track sheets.
c.) I still use disk warrior in theory, but I haven't really used it yet with this setup (had it for 7 months now)
d.) For EQ I use a combo of all the URS stuff depending on what I want to hear. For compression I generally don't use plug-ins. If I really need to the Chandler TG1234 plug-in isn't bad for vocs/overheads. And actually the URS 1980 is great for deessing.
e.) I have 4 gigs. And that seems to be treating me well.