Before you waste money on AMD, check out the i5 by Intel. They are faster for the most part than the phenoms (especially for gaming), and very comparible in price. I got my i5 for 149.99$ at microcenter. The new 1156 motherboards are very cheap also in comparison to the older i7 boards.
I got a MB, 4gb 1600 DDR3 OCZ ram, and the i5 for 340$. MUCH better than anything you can build from AMD for that price, especially since AMD still uses DDR2 (if I remember correctly).
1) At the resolution you're running at, and for the uses that you stated in the OP, you don't need to spend more than $100 on a CPU. Anything more is overkill. The cheapest i5 seems to be $200 on Newegg, and that won't give you a significant performance boost on games over the $90 AMD tri-core that I linked above.
2) AMD boards do indeed use DDR3.
3) As I said before, DDR3-1600 is overkill for gaming (or for anything else except heavy A-V editing or game development or something similarly intensive). Get some 1333 and don't look back.
"'Casual games' are something the 'Game Industry' invented to explain away the Wii success instead of actually listening or looking at what Nintendo did. There is no 'casual strategy' from Nintendo. 'Accessible strategy', yes, but ‘casual gamers’ is just the 'Game Industry''s polite way of saying what they feel: 'retarded gamers'."