Re: X3 Vs. X4
Phenom II cpus are all built on the same architecture, but the X2s and X3s have locked those additional cores (sometimes due to instability but I think it's because it's cheaper than building three separate architectures). Getting the Black Edition of any Phenom II, means those locked cores are now accessible. Certain motherboards and BIOS can unlock them. Easiest one is Asus, which also gives you the option to unlock just one additional core (in the case of of the 550 and 555) in case one of the optional two aren't stable.
If you get a Phenom II X3 BE, you can unlock that fourth core, but you're not guaranteed stability and not every board can do it. Do some research...but I know all the newer Asus mobos do unlock the cores.
And yes, there is a bit of benefit in having an additional core. We're not talking about an extra 50 fps, but an additional 10 fps isn't out of the question depending on what games you're playing. In any case, I'd go with the Phenom II, 720 BE or the 550 BE and then try unlocking.
Desktop: AMD Phenom II 840 X4 @ 3.7Ghz // Asus Sabertooth 990FX // CM HAF 932 // XFX HD 5770 // 8GB Corsair Vengeance @1600 + 8GB G.skill Sniper @ 1600mhz (C9) // Cooler Master 600W // Zalman CNPS9900 // Sidewinder x8
Laptop: Dell Vostro 3555 // AMD A6-3400 1.4Ghz x4 w/ 6520 // 500GB HDD // 4GB Ram upgrading to 8GB soon // Backlit Spillproof Keyboard