The 955 is the same as a 965, except it's 200MHz slower and $20 cheaper. Using AMD Overdrive, you can just turn up the multiplier a notch to change the speed to 3.4GHz, making it the exact same. And FYI, quad-core isn't "better" for gaming, as games don't fully utilize all four cores. For current games, a good dual-core processor will net you performance very similar to a quad-core. Regardless, quad is still the way to go, as they're much better for non-gaming applications. For gaming, the i5 or a 955 is definitely the way to go.
Knowing the following would really help:
- What you plan on using your PC for
- If it's for gaming, which games and at what resolution?
- What are your other specs? (video card, RAM, PSU, etc)
Both are excellent processors and will easily last you 3-4 years without even overclocking. For gaming, they're both pretty even, sometimes with the 955 coming out on top by 1-2 FPS. In Ubisoft games, usually the i5 has a larger lead. For professional applications such as programs for video encoding, ripping, compression, etc - the i5 will generally finish tasks about 5-10% faster, which will save you anywhere from a few seconds to a few minutes depending on the length of the task.
In terms of overclocking, both are good. At higher frequencies, the i5 is a bit faster clock for clock, so it generally has overclocks that are more powerful. You'll easily hit 4.0GHz with either processor, assuming you get the C3 version of the 955. The AM3 platform is probably a little more stable and will support new CPUs for another year or two. The 1156 platform isn't quite as developed since it's much newer, but it still appears to be fairly solid (putting aside the Foxconn socket issue). The future of the 1156 socket is really unknown - but honestly, I can't imagine you needing or wanting to upgrade either of these processors in the next couple years. Note that the P55 sockets for the 1156 platform have had burning issues with higher overclocks, though it supposedly has been fixed.
An i5 platform will cost you about $50-$60 more, but will also save you $10-$20 per year on energy cost, assuming you leave your PC on 24/7. The cost will probably even out over a few years. The i5 is more power-efficient, but also gives off much more heat and as a result, probably is a tad bit louder.
Basically - both are damn good.