if you get amd athlon ii x3, add a cooler in case you overclock (you'll need to, later), and something like ASRock 970 EXTREME4 AM3+ AMD 970 SATA 6Gb/s USB 3.0 ATX
(afaik, the only 970 chipset mobo that supports x8+x8 cfx/sli). check with your local shop/website for availability. you can use the single card now. if you want to add another card later, you'll have the option to do so.
for gaming, gfx cards take priority, so the pentium+5770 combo.
google crysis benchmark with the cpu name.