I would definitely say the dual core as my 4200 has been running PERFECTLY. No matter what I chuck at it, it just eats it up and spits it back at me. I can run all my health checks (spyware, adware, virus and defragent my hard drive) while using say Dark Basic Pro, or playing a game with absolutely NO slow down whatsoever. I have no managed to get it above the 50% useage mark yet
On the gaming side of it, I can easily say that it will run extremely well if you team it up with a decent graphics card (i.e. the 6800 or 7800 series from nVidia, or the X850XT from ATI).
Multi-threading technology will become more widely used soon and so the dual core will stand you in good stead for the future