I think the Opteron 185, FX60, and X2 4800+ were among the fastest. You will want to check the manufacturer's website to see what CPUs his board supports, and if he needs a bios update.
I have seen 939 X2 CPUs in the FS/FT forum lately, so that would be a good place to start. The prices on ebay and such tend to be inflated/gouged.
I am just now replacing a 939 X2 3800+ system I built summer '05. Really only doing it to get hardware virtualization tech. I got the Gigabyte 785G based US2H and a retail AthlonII 240 for $102.99 shipped from Newegg. Your dad could grab a combo like that and add 4GB of ram for $90. I just could not pass up a significant upgrade, for under 2 bills. I had bought a new PSU last year, so you might need to factor that in if he decides to spring for a new setup.