upgrading X51 R2

Hi. I am thinking about upgrading my X51 R2 (bought in december 2015) in the near future and I am looking for some advice on what to choose, since this is the first time I do this on my own. I have found some answers, but I am still unsure about it.

My current cpu is a i5-4460 3.2 GHz and I have a GTX 745 GPU and have a 240W power supply.

 What I want to know is what i7 CPU's are compatible / recommended.

Samething for my Graphics card. Regarding graphics cards, can it handle double slot cards? If so, which?