I'd be looking at the Intel Core 2 Duos or quads. I've got a Core 2 Duo 6600 (2.4 gHz), and it is smoking fast. They are affordable as well. I know you are used to seeing gHz above 3, but this is actually faster than that Pentium D.
http://www.newegg.com/product/produc...82E16819115003