Yes you can but prepare for very annoying lagging. You processor is under minimum requirements, more RAm is needed and believe me it's not fun when fps is around 5 - 15.
I play with Pentium 4 3.0Ghz, 1 GB RAM, ATI Radeon 9800 PRO, Windows XP Pro. Single player plays great as long as my setting are set to medium. Mutliplayer isn't too bad either.
You might be ok with your specs. RAM might be a slight issue due to the fact that you're on Vista.
COD4 has been known to run great on low spec systems but up to you if you want to roll the dice to see if it'll run on your system.
Your processor is slow, but since it is a duel core I bet you'll be fine. The biggest holdbakcs will be that you only have a gig of ram, and the 6150 is a pretty bad performing card.
With the specs in my sig, I can play the Demo on highest settings at 1280x960 with around 25+ fps, but usually around 30 or above for an average.
