im thinking bout gettin a aem fuel rail an a skunk 2 manifold for my stock ls motor.. what kind of performance gains should i expect an also which one would i get the most gain from

 

You won't gain anything from the fuel rail. Unless you're pushing some serious power to where you need the higher volume of fuel, it's not going to do anything but leave you with $150 less.

I'm not sure on the intake on an LS, but when the crappy GSR's are replaced, I've heard of 10-20hp gains.