Both Great Games
If you are looking for better graphics, I would recommend COD4, of course. However, there actually still seems to be more servers that cater towards COD2, and most players that I have encountered agree that COD2 is the best of the series. I do not own COD4, but have heard many glowing reviews. However, the single player story-line is extremely short in COD4. I have had some problems with COD2 running choppily on my nVidia iMac, so you might actually get BETTER performance from COD4 if you have a newer computer. Anyway, it's really up to your preference. I recommend the Tribulation Force servers, who also have a good web-site where you can check your stats. If you get COD2, keep a look out for [The Long Patrol] Jester on Trib-Force! Good luck, and have fun.