Of the games they tested, only Oblivion takes advantage of dual core. Q4 and COD2 also benefit from dual core after SMP enabled patches were released, but they weren't tested. From what I've seen, Q4 is pretty CPU intensive and definitely gains from dual core, but COD2 only sees noticeable gains at lower resolutions where the GPU isn't the bottleneck. At GPU limited resolutions there is only a 5% performance gain at best.
For the vast majority of games out at the moment, your A64 @ 2.5GHz is more than enough, especially since you are running a single GPU. In single threaded games, it will be very close to the numbers of the X2 5000+, which runs at 2.6GHz.
However, if you want to 'futureproof' your rig, then dual core is definitely recommended. FWIW, Crysis (surely you've heard of it, right?

) has dual core CPUs as part of the 'recommended' spec - a sign of things to come perhaps?
If you are running a S939 rig, it doesn't hurt to upgrade now, prices are great and stock will run out soon on S939 CPUs. Not a problem if you don't mind buying 2nd hand stuff, but if not then I suggest you get a move on before S939 stock runs out.
