Help recommending computer parts please

[insaneownage]

I want to buy Call of Duty 4 and be able to play it with pretty decent graphics. Not necessarily breathtaking quality, but I want it to look pretty good and go pretty smoothly.

My computer is an HP Media Center m7360n. I haven't changed or added anything since I got it.

What part(s) need I buy to do this?

I appreciate the help.

 

[Prometheus]

I find anything not custom, is not very practical for upgrading.
Do you have a budget, and have you ever built a computer?

 

[insaneownage]

I'm not really looking at buying a new computer. I've never built one. I'm just wondering if it would be possible to get decent graphics with what I've got.