Considering you said that you could get them for the same price you have to ask another question. Do you plan on upgrading components around the motherboard down the road? If so go 955, if you don’t do i5. Your situation is a bit different than mine, for me there is no point in look at the i5 it is either 955 or i7 because in my area the 955 setup is about $50 cheaper than the i5 and the i7 is about $100 more than the 955.
You also mentioned overclocking: I have heard of both going over 4ghz. With that being said i5 starts lower. You also don’t get the same from overclocking them both one scales a little better, but that is another argument that I personally would not like to get into.
i5 is a higher quality CPU, generally does better in benchmarks and while they both OC decently well, i5 can OC a decent amount without breaking a sweat. In my case, I'm at 3.68ghz with turbo on (well, enabled from C states but not actually on), meaning 1 thread goes up to 4.2ghz, 2 threads at 4ghz. If it's the same price the i5 is the better core, however, for gaming you probably won't see a difference one way or the other unless getting a 5970 or crossfire/sli the high end cards. So then it really depends on the other components, compatibility, and if you need to buy new things cost as well.
The locked multipliers are known to be 'harder' to overclock because they force you to overclock your ram/NB at the same time. Where as on a black or extreme edition their unlocked multipliers allow you to overclock the CPU seperate from everything just by changing the multiplier.
IMO i'd call the i5 easier considering the massive amount of headroom it has starting at such a lower clock than the 955's 3.2.
With 5770 crossfire I doubt you'd notice a difference between Phenom II and i5 so take your pick. If the i5 is significantly more expensive go for Phenom II. If not take the i5.
I wouldn’t say that the i5 would be hard to overclock, but it will take a little more work than the 955. With crossfire you see little to no difference between the i5 and 955 don’t buy into certain peoples hype…
Pretty much if you are mainly going for gaming; buy the cheapest chip out of the 955 and i5. They are both strong performers and both overclock well.
For 3.8 you'd likle have to turn off C1E/Speedstep (thermal controls) and turbo. Then up the cpu/ram volts and move bclock up to 3.8 and adjust ram ratios accordingly. However I have very little experience overclocking newer intel chipsets.
Any questions about overclocking you have, post in the overclocking section and I'm sure you'll get all the help you need.
i5 is very easy to overclock to 3.8 ghz even 4ghz - but only if you don't mind exceeding Intel's specs for maximum VTT voltage, which is 1.21V. With a BCLK of 175 I have 1.197VTT and 3.68ghz very stable, turbo mode enabled as well so it can reach 4.2ghz with just 1 thread operations. If you want to go higher, a 200bclk needs about 1.28-1.3V VTT or so, but then you can easily hit 3.8+ghz. Any OCing takes a lot of time if you want to do it right, no matter which core. Check out some OC guides for each core if it's really important to you.