The X3 Phenom II's blow the X2 7750 out of the water. There's no competition between the two. The Phenom II is on 45nm, it overclocks very well, and uses less power, yet performs better per clock, and is higher clocked to start.
I was looking closely at the AMD 7750, and it looked like a steal! But it is made from a 65nm Phenom. So the ram runs at DDR2 1066, which is an improvement over the old earlier 65nm and 90nm AMD AM2 X2's 5000's and 6000's, but one drawback. Everything I've read says the top overclock is 3.1ghz. I've heard its an awesome performance/price. Which is fine for some games, and will rock your old games, but limited for future use. You will see the difference in games like Crysis when compared to the Phenom II's and i7's.
I thought the AMD X3's where a laugh. I thought they were gimpy, until I bought one. I just bought the AMD X3 720 BE (unlocked), It runs smooth and stable at 3.75ghz, temps stay under 44'c under load, and in-game I'm getting +30fps playable in Crysis & warhead @1600x1200 HIGH/Gamer with an old 9800GTX! It really does play as fast as a Intel 920, or AMD 940 (within -7fps). In old games it getting over 100+fps on high settings. Up from 40fps with an overclocked 939/9800GTX setup. My 3dMark05 score went from 12000 to 24000! 3dmark 06 from 7000 to 17000!
I've been reading EVERYTHING about the new quad cores, I've wanted one for over a year, since Agena, but this X3 honestly is FAST in-game. It may not be quite as fast compressing videos, but for gaming is running 95% or more as fast as the quadcores, but only $145!
I thinking about returning it, and upgrading to the 940, but there is no need. It really runs fine with everything I've been throwing at it for the last week. If your a gamer like me, your games will play just as fast as a quad, but you'll have $80 left in your pocket instead.
If you get a motherboard too, and have to format your harddrives anyway, buy a 2nd identical hardrive for a RAID 0 setup. That made a difference too this build. The ram I used was Patriot Viper, newegg has it for $59/4gigs, and its running at 4-4-4-12/800mhz or 1066@5-5-5-15-2T for 1600x1200 res on up. That was a good deal too.