Would it be a cost effective to upgrade – from a pure gaming stand point probably not.
FPS games performance is more a function of the GPU and unless the CPU is bottlenecking (Don’t think his is), then upgrading the CPU has limited value.
I heard that to be cost effective you need to move up the latter at least 3 tiers. You are right at this point.
The Phenom 2 X4 955 is in the same group that contains the QX6800/50, the next tier up contains the i5-750 and i7-965 (NO amd challengers) the highest tier is the new SB CPUs (not shown in the link.
Based (solely on the CPU) this is why I’d say toss-up, but for only gaming probably not cost effective.
What may make it cost effective are the advances in feature of the MB.
Would be a good Idea to wait for BD to see what it brings to the table – Wait for REAL benches.; However if you wait another 6 Months you also have Ivybridge as a choice.
Ref for tiers (there is a newer one out):
http://www.tomshardware.com/reviews/best-gaming-cpu,2599-7.html
As far as OC, yes that will boast HIS cpu performance, but should not be used for comparing AMD (Oced) to a non OC Intel CPU – Keep apples to apples please.
Yes he may be able to unlock the extra 2 cores, but currently that will do nothing for current games – Again the stress here is MAYBE and Will that IMPROVE gaming experience.
Disclaimer: If I was a gamer, and then the platform would not be an issue – go with which ever performs better. I’m not a gamer and prefer the Intel platform – The CPU is only PART of that equation. It more a question of the monkey vs the 800 pound gorillia. While the monkey may pull a rabbit out of the hat, I'll bet on the 800# gorillia.