I've built two systems for friends & family using the Phenom II X2 (just for basic office work & websurfing). I also build one with an X3 Phenom II. While the motherboards supported unlocking cores I've never found any extra cores that were stable enough to run at full stock speed.
NOTE: this was about a year back; BEFORE the new Athlon II X4's came out. Now I use Athlon's exclusively for basic builds.
Could just be my bad luck....
But, I'd go the few extra bucks for the 955 if you can then you're sure you have four(4) solid cores. And, you can likely OC them to 3.6 to 3.8 Ghz with no trouble at all.
You can OC the Athlon chips too, just takes a little more knowledge of OCing......
I've been OCing for quite some time since Celeron 300A's and chips like the Athlon XP 1600+ with the multiplier unlock trick were the kings of the roost nowadays it's brutally easy to do lol.
I'm basically building this to play Crysis 2,BFBC2 (which my laptop plays but has weird lag spikes sometimes it'll run silky smooth for an entire spawn then I get killed and respawn and it lags like hell) so I'm looking to go probably full on quad then to even things out.
You'll need some serious GPU(s) to play Crysis 2. I'm playing the pre-release version at 50--60 FPS @ 1920x1080 all settings max'd out. I'm on the Crossfire 5870 system listed in my profile for that.
A 5770 has about 1/4 the power of that system.... will leave you a bit short.
If you can swing a few more dollars, pick up a 5870 as they are on some great close out sales at present... (should be playable at lower settings)
If you have about $500 get a single GTX 580 (that's even more powerful than my dual 5870s)
Your other option would be to upgrade the motherboard a bit to allow support for dual GPU cards in the future. I still would not buy a 5770 for Crysis 2 thou as even adding a second one will only get you to 5870 power and cost you a bit more to boot.