Your analogy of dual core is very wrong.
Firstly i'd like to agree with Ren - if you don't upgrade because of 'the next thing' then when will you? you'll miss out on all the goodness of a fast machine until then. THen when it comes along, sell the old box to a student and buy the new one. I was considering upgrading recently, but i've lost the interest to, i would sell off my old 3200xp 2k4 rig in order to make a starter block for a 939 system that could go up to multicore & multiGPU. Like Ren did, you can buy a great system now that will do you well with multi* upgrades.
What i *am* waiting for, however, is the ATi answer to SLI, it could change mobos. I would also like a BTX system (if not a sff one) which will be more ergonomic... sadly i have to wait for them to come out - but Dell have jumped on it so it won't be long before OEM boards are BTX.
On to the post by azcn - multicoring will give great benefits to the smoothness of gameplay. Once things are threaded and scheduled properly then you'll find that, as with HT intel chips, you'll never get 'slow downs' in the input of the machine, they'll become far more real-time. The ability to spread the load will allow for smoother gaming overall. It will also increase the speed that most things are done, if not drastically then certainly well enough to be worth it.
The biggest benefit will be, imho, insteat of 2600MHz running one chip on all aspects of the game, you can have (say) 2000MHz focusing on world rendering & interface with another 2000MHz focusing on physics and AI. While code that's not enhanced for this type of multistuff won't benefit from it, it will eventually happen.
It's a moot point about upgrading to multicore. It'll happen and it will benefit you.