I can give you some limited experience I had with a slower machine.
1.6Ghz Athlon XP, 768MB DDR Cas 2-2-2, stock 9700 speeds. 11,890 for a 3dmark2001 SE score. Flyby on UT2003 was 60 FPS, 35 FPS on botmatch (Im really digging in my brain for the exact numbers). Not even sure on the resolution I used for the UT2003 test. Believe that was demo version also?
I upgraded recently. 1.8A @ 2.61 Ghz, 387Mhz DDR 2-2-2-5T stock speed. 3dmark 2001 15,080. Never have bothered to redo the benchmark on UT. On capture the flag citadel, 1280x1024x32 4X AA, 8X AF, high details loaded for everything I can average 45 FPS using fraps. I really should just redo the bench to see what I really gained for sure.
As others have said, Anand did a pretty good testing of CPU scaling. Check those out also.