I want to build a new system on a tight budget ~400 (not including graphics, DVD, and PS). I plan on gaming down the road (summer vacation), but in the meantime, web-browsing, school work, and multimedia are my main concerns. Also I heard that the Athlon II's don't fully utilize ddr3?
Athlon II is the same architecture as Phenom II, it uses the same memory standard and has the same memory controllers. I have no idea who told you Athlon II can't fully utilize DDR3, but they can't be right.
Personally I would go for the X3, the world is moving towards multicore, and many new games utilize more than 2 cores. You'll also experience better multitasking and better performance in a wide range of applications too.
+1 for the X3. The main difference between the two is that the Athlon II's have no L3 cache whereas the Phenom II's do. However, like loneninja mentioned, the extra core in the X3 will have a far greater impact than the L3 cache. I use an Athlon II X4 and its a wonderful chip for what I use it for. Would highly recommend.
I'd go for the x2 for gaming and probably try to unlock the cores. The l3 cache helps in games and if you manage to unlock the other two cores, though you should never count on doing so as it's hit or miss, then you will get some good extra performance. Without unlocking though, the X3 will definitely serve you better for every day tasks.