Well 100£ is pretty good for and 8120. For the price to upgrade to i5, you could get a pretty beastly cooler, and probably match, if not beat the i5....., and if the 8 cores are also good for heavy multitasking.... just my opinion.
As far as I understand, an FX 8150 won't support 2000Mhz RAM.
7770's are very good in crossfire, but if you havent bought this PC yet, dont buy 2 cards, buy one. I think the 660ti is roughly the same price as 2 7770's.