Cannot decide for my first build

I am having a hard time figuring out what I want for my first build. I am on a budget, but I'm not afraid to put more money into the CPU. The problem is, I don't know which one is going to give me more bang for my buck. I am referred to benchmarks all the time, but it doesn't answer the two questions I want. How long will *this* CPU last me before I need to upgrade and how will it handle video games(high, low settings, etc)? I see the number difference, but I am not computer savvy so I have no idea what they exactly mean. Like the Phenom II X4 955 could be perfect for me while the i5 2500k is overkill.

The CPUs I am looking at are the Phenom II X4 840, Phenom II X4 955, and the i5 2500k. Once I figure out which CPU is best for me then I can finally start the purchasing of the components.

CPU- Undecided
GPU- Radeon 5770 to start out with(won't buy Nvidia)
RAM- 4gb DDR3
HDD- WD Caviar blue 500gb
PSU- XFX 650w
Mobo- undecided
Case- whichever is cheapest

Approximate Purchase Date: ASAP


Budget Range: $500


System Usage from Most to Least Important: Gaming


Parts Not Required: Monitor, keyboard, mouse


Preferred Website(s) for Parts: Newegg, Microcenter, Tigerdirect, Fry's


Country of Origin: US


Parts Preferences: by brand or type: ATI GPU


Overclocking: Maybe


SLI or Crossfire: No


Monitor Resolution: 1280x1024, however I am buying a 1920x1080 but I don't know how soon that will be.


Additional Comments: Quiet PC