Phenom II 945 was originally released as a 125 watt part. It has been recently reissued as a 95 watt part. Check the "TDP" rating of the part you are looking at.
Both are overclockable thru "bus speed" setting (in fact all are).
The 955 (and others) however is a BE "Black Edition" which means it has an "unlocked multiplier" so the "bus speed" x "multiplier" = clock speed. This makes for easy simple overclocking, without getting into manipulating bus, voltages, etc. So you can easily add some extra speed without being an extreme oclox expert.
Out of the box, stock clock for 945 is 3.0Ghz. For 955, it's 3.2 Ghz. Both have 8 meg cache. Both Quadcore.
Apart from that, the newer issue of 945 uses less power, at stock clock (95W).
Otherwise, these 2 cpu's will perform similarly.
If you plan on oclox, get the 955; if not, the 945 will suffice.