With Socket 478 you can only go up to a P4 3.4GHz (yes there's a 3.4GHz Prescott that will fit in the socket). Core Duo is out of the question on your current setup. Higher processor speed is not really a relevant issue especially in the old NetBurst processors. So instead invest in more RAM at higher speeds (DDR400). I know having a lot of VSTs open takes up memory very quickly.

As far as video card, the best AGP card on the market right now is the 7800GS. You can get it for under $180 at newegg.