Start looking at porting your work to dotNET *now* rather than later (should have started a couple of years back, actually, but if you only have 2004 not much you could have done). Serious speed issues have been reported with VBA under 64-bit.
And, for what its worth, AutoCAD still doesn't do much of any multi-threaded processing. Its pretty much limited to regen/redraw, and any multi-threading from the user-side can't involve anything AutoCAD.