I do not know of any benchmark dedicated to Call of duty 2 so i used fraps and my phone to take a fewpictures (sorry for the quality). You can clearly see CPU utilization hovers in the 70-80% with the Athlon 64 X2 4200 so the second core gets some work to do:

I also tested 3 games that are single threaded only: Splinter's cell: chaos theory; Need for speed most wanted and Battlefield 2. There are no difference in performance between the Athlon 64 3500 and Athlon 64 X2 4200.
I think that, after a few more games tested, 2005 is the very last year where you could still play anything at the highest possible details with a single core Athlon 64! It is amazing how fast single cores became obsolete! By 2006 you could not possible play most games at max details with a single core Athlon/Pentium 4.

I tested a few games on my Phenom II X4 3.2Ghz:
- Need for Speed Carbon (2006) - only utilizes about 26% of 4 CPUs, so basically just one cpu. It stutters a bit on 2.2-2.4Ghz single core, needs probably 2.6-2.8Ghz single core. You will need the strongest s939 CPUs released.
- Test Drive Unlimited (2006) - only utilizes about 35% of 4 CPUs, so it can't fully utilize 2 cores. It will not run well on 2.2ghz dual core. Highway driving is very demanding in this game. On Phenom II I see drop to about 60 fps, so you can expect about 40 fps. You will need the strongest s939 X2 CPUs released.
- Microsoft Flight Simulator X (2006) - when set to ultra high quality, it seems to run at about 15-20 fps. It doesn't seem to benefit from more than 2 cores. I would recommend at least Phenom II X4 3.6-3.7Ghz for this due to clock speed. Your best bet these days is the Steam edition, supposedly a recompiled Gold + Acceleration pack. Currently costs 4.99€.

You will have a really hard time finding 2.6Ghz X2 Opteron CPU (OSA185DAA6CD) or FX-60 (ADAFX60DAA6CD) for s939.