Dual core processors have been widely available for some time now, but many have yet to make the upgrade. Where price could have been a factor before, today that's no longer a barrier with CPUs going for as low as $70. So rather it's become a question of how long you want to wait to upgrade your computer in general.
Depending on what games you want to play in the near future, that time might come sooner than we'd expect. It seems that some of the first titles to require dual-core CPUs are now approaching.
Assassin's Creed is the first title to earn such an honor, with both the developers and hands-on reviews showing that the game does need at least two processors to sate its clock-lust. The figures show some pretty amazing results, with the difference between a single core system and a dual core system (or just disabling the second core for testing purposes) being dramatic. This is in stark contrast to many games in recent history, where the GPU has played a much more important role than the CPU has.