The i5's aren't that much better than the X4 955. To be honest, you wouldn't notice any difference in game.
The other problem I have with getting the i5 is that the best board for it (Asus P7P55D-E Pro) is currently unavailable from Newegg.ca, so I don't know if that's a big problem or not. Here it is from DirectCanada for $201. With the ir-750 from Newegg for $209. Total for the build is then $1,266.
EDIT: Forgot to add that if you go with the i5, there is literally no upgrade path available. Intel is moving the LGA1156 socket to the mainstream/low-end market, so the i5-750/i7-860 are the most powerful CPUs that you will see for that socket. So when it comes time to upgrade the CPU, you're building a new computer.
I wouldn't say the i5 is only as good as the X4 955. The i5 is better, but for gaming, the CPU doesn't really impact that much. However, the AM3 socket does have an upgrade path, though no one knows how good that will be.
Intel is the best right now in terms of performance. However, they are basically non-existent in the lower and mid range budgets.
I'm not familiar with that acronym, but I'm 100% sure it will. This build would play Crysis on max details at 1900x resolutions. There isn't any game that's more demanding than that.