Whic motherboard and Processer Should i get?

Well I would personally recommend the Intel Core i5 750 CPU for most new PC's since it hits a certain sweet spot on the CPU market and also you may want to check that the GTS 250 your buying has DX 11 support since most GTS 250's only have DX 10. Also yes I believe that the 750 watt is an overkill but if you can get it at a good price there is really no downside.