The i5 2500k is better for nearly everything (with the exception of applications which actually use all 6 cores of the phenom ii 1090t effectively and even then, it is very close).
For gaming, definitely get the i5 2500k.
If gaming is you first priority then the I5 2500K is the way to go. If you are uncertain if you really want to upgrade now wait til the end of year when the newer generation of CPU's are supposed to be released.
...I don't know why you threw the i5 into the mix and then restricted the upgrade to $100.00 as that is simply going to cost way more than that.
As for replacement of the quad with a hex I, as an owner of a 965 and a 1090T, can tell you that for run of the mill operation you'll not see any real advantage.
It'd be something you'd only consider if you were making money from your work and you end up with more product to sell in your hand at the end of the day and that I think would be the bottom line.....
The 955 of yours is most likely a BE processor so why not download AMD's own OC software and increase the performance that way, it's fun and it's free. If download theur Fusion tool you can stipulate the environments so that you can allocate increased resources of you pc for game play where you need it. Catalyst also has a tweaking tool for the GPU as well so there's a third option and you haven't spent one cent....