Who said that Phenom II and Athlon II (AM3 Processors) don't work on AM2 or AM2+ motherboards. Yes they work just fine. But the opposite is not true. You cannot plug an AM2/AM2+ CPU into a AM3 socket mobo. This is because AM2/AM2+ socket is based on 940 pin Pin Grid Array whereas AM3 processors are based on 938 pin socket type, hence making it impossible to plug a AM2 processors like Phenom 9850 to an AM3 board.
The motherboard you mentioned, ie. Asus m2a-Vm based on AMD 690G chipset and supports Phenom II and Athlon II CPUs with a Bios update. Check here for the CPU support List of your mobo.
Go for a AMD Athlon II series CPu now. Don't spend much money to Phenom seroes as AMD is going to relaease their new gen processors in the end of 2010.