Encoding video (avi, mpeg, etc) into wmv, a major reason for upgrading, is not all that much faster. The system is more steady during the process, but the actual operation hasn't sped up much.

It is possible with this one that the bottleneck is hard disk throughput rather than CPU time. If that is the case then unless your new disks are dramatically faster you won't see much improvement.

How much RAM do you have ?