MAKE SURE YOU HAVE THE RIGHT POWER SUPPLY OR ELSE YOUR PC WILL BE FRIED UP!
ok thats over with...
* You could run CoD4 on Medium/High Settings (Mostly on Medium for smoother gameplay)
* You could play UT3 and Orange Box on Medium/High Too.
* I would say your set up right now is great but some PC gamers are readying for the Nvidia 9000 Series
* Your set up will last you about another year to play all the games on Medium/High
* You can last 2+ Years if you have your set up on low settings during gameplay
BTW: Are you running Windows Xp or Windows Vista?
Hehe thats why I quit computer gaming... I hate upgrading PC Video Cards, too expensive but its your choice. =]