From my research recently looking at upgrades myself, there are large performance improvements when going from original Phenom to Phenom II. Anandtech did at least one article on the topic, the architecture and on-chip cache design is apparently much much better with the Phenom II.

Clock rate is one variable and it doesn't necessarily correlate to real world performance, lots of variables can affect that, so no you don't want to just buy the biggest number.

Likely to be buying a quad core Phenom II 3.0 GHz later today.