To the OP, welcome to the world of First Person Shooters. If you think you like WAW you really gotta follow these guy's advice here and pick up COD4 which in my opinion is far superior of the two. Infinity Ward > Treyarch.
BattleField 2142 and Rainbow 6 vegas both had good leveling up systems and nice new gear, Resistance 2's co-op is alot of fun with very powerfull weapons at later levels. COD4's early level weapons are pretty much just as lethal and effective as later level weaps.
But I think all players should have access to all weapons that should all be balanced properly, and to show your status of a higher rank with just more options to customise how your character looks.
Halo 3 has a good ranking system. I look forward to seeing what Killzone 2's perks, levels and gear ranking system is like.
So yeah, pick up COD4, and if you're really new to fps games, play everything Valve has ever made :D
