so which gpu should i pick im pretty clueless becuase this is going to be my first rig
Go for PSUs of well-known companies and have at least an 80+ certified rating. Some examples of good companies are- Corsair, Cooler Master, Antec, Silverstone, Seasonic, Gigabyte, EVGA, Fractal Design. A fully modular PSU is one whose cables can be completely removed as per will, a semi-modular is one which have some removable cables, and a non-modular one is whose cables can't be removed at all. Modular ones are the most expensive, semi-modular ones are slightly cheaper and non-modular ones are the cheapest. As far as the GPU is concerned, memory (Measured in GB usually) isn't the best thing to compare GPUs with. A 2GB GT640 has the same memory of a 2GB GTX690, but the latter has a significantly better performance and is a lot more expensive. I'd look at something like a GTX660 for te Nvidia side and an R9 270(X) for the AMD side. Nvidia cards have better PhysX (Check out this video to learn more- http://www.youtube.com/watch?v=-x9B_4qBAkk)
and certain AMD cards are getting Mantle which will be boosting performance in certain games (Like BF4) by up to 50%. Hope this helped.