Buying CPU

So I'm building my own gaming computer for the first time and I really don't know much about it.
I haven't decided on the motherboard yet, I have a graphics card and power supply already, and I've picked out my case. So now I need to deiced on a CPU.
I was going to go with the CPU suggested in the May 09 gaming computer, but after reading some comments I started to look around some more. After looking around I fell upon three AMD processors that seemed like a good buy, but since I don't know to much about CPU's I'm not sure which is the best pick.
The CPU's are-
AMD Phenom 9650 quad core - http://www.newegg.com/Product/Prod [...] _-19103288

AMD Phenom II X2 550 duel core - http://www.newegg.com/Product/Prod [...] _-19103680

AMD Phenom II X3 720 triple core - http://www.newegg.com/Product/Prod [...] 6819103649

All of these seem very nice, just not sure which to get.
I'm also open for suggestions, if you think there is better CPU for around $120 then please feel free to say so.

+1 for the 720

+3 for 720.

At any rate, what's the rest of the specs?

For the 720?

-Model

Brand AMD
Series Phenom II X3
Model HDZ720WFGIBOX

-CPU Socket Type
CPU Socket Type Socket AM3

-Tech Spec
Multi-Core -Triple-Core
Name -Phenom II X3 720
Operating Frequency -2.8GHz
Hyper Transports -4000MHz
L1 Cache -128KB+128KB
L2 Cache -3 x 512KB
L3 Cache -6MB
Manufacturing Tech -45 nm SOI
64 bit Support -Yes
Thermal Design Power -95W
Cooling Device -Heatsink and Fan included

I haven't decided on a motherboard yet, thats still pending.
http://www.tomshardware.com/forum/ [...] cent-price
Graphics card is a EVGA GeForce 8800 GT.
Power Supply is a BFG ATX12 2.2 450 W.
The case will be a Antec 300 or a AeroCool Vx-R .
4 gigs of G.Skill RAM.
A Seagate Barracuda 7200.12 500 GB(plus a 320 which I already have.)
And I haven't deiced on a cd/dvd drive.
As for a Monitor, I'll probably be getting a new one. I'll ether be getting a 28" LCD HDMI or a 24" LCD HDMI, cause a 28 might just be to big.