As said above Core 2 Duo is the best. Pentium and Celeron Dual Cores are below the Core 2 Duo but better than the Pentium D, they are cut down Core 2 Duo chips. The Pentium D is the worst, it's basically two Pentium 4s glued together, it is infamous for running very hot, and being a very slow CPU overall, even with the really high clock speeds.
Which ones you can get will depend on which LGA 775 motherboard you have. Check the CPU compatibility list for your board to find out which CPUs you can use. Note that a lot of the higher performance Core 2 Duos will be quite pricy as they are no longer manufactured, but are in high demand from people seeking to gain more life from their older systems. If you are looking for high CPU performance, it probably will be more cost effective to look at a new build. Current dual core CPUs range in price from $50 to about $120. Some of the higher end Core 2 Duos will sometimes sell for $200.