You'd be much better off with an AM3 CPU, AM2+ CPU are older and thus do not perform as well. AM3 CPUs can be used in some AM2+ and even a few AM2 motherboards. I know that AM3 CPUs don't suffer any performance issues in AM2+ motherboards, it may be different for AM2 which has a lower HyperTransport bus speed.
AM3 processors support DDR2 and DDR3 memory, and thus will work in any AM2+ or AM3 board.
AM2+ processors only support DDR2 memory, and will only work in AM2+ boards, and AM2+ boards only use DDR2 memory.
AM2+ was kind of a stepping stone to AM3 in an effort to allow upgraders some extended life out of their platforms without buying everything new. If you are buying all new, go all AM3 and DDR3 memory most definitely. If you have an AM2+ board now, you can move up to the new Phenom II's processors and keep the same board and DDR2 memory.