AM2+ motherboards in most cases support AM3 CPUs after a BIOS upgrade (which should be performed before switching CPU!). AM2 motherboards often, but certainly not always, support AM3 CPUs. Motherboard manufacturers usually have a list of supported CPUs on their webpage. In your case, it is available here
Unfortunately, this appears to be one of those rare cases of an AM2+ motherboard not supporting AM3 CPUs :doh: (Though considering it is from one of the crappier manufacturers, probably not that surprising). The fastest supported CPUs are 95W TDP Phenoms (as opposed to Phenom II), which are not quite regarded as one of AMDs more successful designs...
The IPC (instructions per clock) performance is higher on the Phenom II than that of the Athlon 64, due to increased cache and various improvements. It also has better performance / watt (especially compared to the 90nm Athlon 64), mainly due to a process shrink to 45nm. Keep in mind though that if you go for one of the most powerful Phenom IIs, you need to check that the motherboard AND power supply can handle the power requirements.
Performance with an older motherboard, however, will not match what you'd be able to achieve on a completely new system since old chipsets have slower hypertransport links, and old motherboards tend to be fitted with slower memory than what is available now.