Im building a pc for a cousin and i was wondering if i would buy a mobo:ASUS F2A55 , CPU:AMD Vision A6-5400K Black Edition 3.6GHz and the GPU:Gigabyte Radeon HD6670 2GB DDR3 128-bit . I was wondering i can crossfire the Grafic processor from the Cpu with the GPU . and with additional 4 GB of RAM what Games can it play?
You cannot crossfire that cpu with that card, only the 6570 which is weak.
In my opinion for really low budget gaming, you're better off with a pentium G8xx, a h61 mobo and a much better card, like the 7750/7770(at least).
No, SLI only available on mid-end mobos(depending on the cpu you'll choose, diferrent sockets), the lowest and best value cpus are the pentium G8xx series, they usually are cheaper than a x4 phenom II from amd(am3 or am3+ mobos), the h61 mobos are the cheapest for those cpus, also there's the b75 mobos which are usually cheap.
As for the cards, it's not recommend anyway SLI/Crossfire with low end cards, so just get a single one like from amd the 7750/7770(lowest priced cards with decent performance) or from nvidia the 650 gtx or 650 ti(which are more expensive, and can't do SLI, cause they got no SLI bridge).