<< Does it help reduce CPU usage? >>
Well, it for sure does if you make use of IPSEC otherwise that 3XP processor doesn´t do any use at all! The difference between 3XP vs non 3XP is just that IPSEC encoding is done by the 3XP instead of the cpu so basically: No ipsec on your LAN = no use of that 3XP.
On the other hand: Quality NIC´s reduces cpu utilisation compared to cheapo stuff... But thats an other story...
Cheers!