Just think of it this way RedUK. You will be running all background applications (like Windows) on one of the processors, leaving you a full 2.2GHz processor to run the game So it will perform on gaming the same as another 2.2GHz AMD processor (which is on the same level as as the 3700+ San Diego). You also get all the multi tasking abilites of the processor, and it is way more future proof in terms of gaming (due to dual core gaming about to take off). So if you have enough money to buy either a 4200, or a single core from AMD, then there is no competition really.
The only single core that challenges the X2s is the FX series (which having a starting price of around £550) and they will outdate themselves quickly.
Personally, your setup before sounded awesome, and I wouldn't change to go back to a single unless you can afford the FX series (which if you can, you might aswell go for an X2 4800 which will perform around the same level, but has the advantage of multi tasking).