My socket 939 is overclocked to 2.45mhz, and does better than the pentium 3.6 (At stock speeds though) Check out the attached pics for my sandra results. I have a radeon 9800 pro 128mb (overclocked to 430/370) and I do 6578 in 3dmark 2003, and 21838 in 2001. If you could overclock a pentium well, you may beat that but a 64 bit processor futureproofs you for when 64 bit software is eventually released.
I heard that prices for the socket 939 components will be coming down soon and the ones that use the 90nm fab should be out soon too. They apparently have a 5% boost in performance over the older 130nm ones. As far as overclockability goes, I would depend on your processor and board. Not all boards have the agp/pci lock needed for those extreme overclocks.