I'm in the middle of my 1st career

and am very intersted in finding a good mod for an already good sim. GWX and NYGM are the biggest names floating around. What are they and how do they differ? Also, I know nothing about installing mods. I only know how to tweak files (like time compression, keyboard configuration, etc).
Lastly, do any of the mods improve the graphics? I have a GeForce 7600 GT and a 3.2ghz processor with 1.5 GB RAM; 1024 x 768 is killing me