The 4000 is nice if you want to do mostly gaming without heavy multitasking, and you can always upgrade that chip in a year or 2 with a dual core (or quad) upgrade (if you go with an AM2 platform).
The dual core is nice if you do heavy multitasking like run heavy duty security suite, have 20-30 windows open or run some heavy cpu tasks simultaneously. the x2 3800 will do ok for gaming with a good video card (like $200 or even $280 for a serious gamer), and can be overclocked modestly for a boost if needed.