I would suggest you don't buy it if you already have MW2. If you're looking to play online, you'll have much better luck with MW2 than you will with CoD4.
It's your choice, of course. I'm trying to keep my opinion of both games out of this thread because I don't want them to affect your decision. If you think it looks cooler, go ahead and buy it. All I'm saying is you'll get more online players with MW2.
COD 4 has a better multiplayer system (dedicated servers on PC platform, not sure about consoles, but the connection is bound to be stronger and more reliable than on MW2), the weapons are pretty balanced, the campaign is original and multiplayer is extremely good. I'd seriously suggest buying it and going from MW2 to COD 4.
Like Dubness says though, many people who bought MW2 have an eye on BFBC2.
COD 4 is pretty sweet, but you have to look at what you want out of it. If you're a big multiplayer player, MW2 is the way to (at least for 360 or PS3, can't speak for PC) the multiplayer in MW2 is MUCH better than COD 4. Single player in COD 4 is pretty sweet though. decisions, decisions..
Yes; I would save the money you have now, get a small amount more and wait for BFBC2 to come out. Console-wise, I would recommend PC, but it's for whatever you decide to have the game on. Accounting how awesome Battlefield has been (especially 2), I would say you cannot go wrong with Bad Company II. If it will have the same concepts and goodness that Battlefield 2 has, then you cannot go wrong buying Bad Company 2.
and was disapointed. I think you cant appreciate it due to it being outdated. so no. Even though it is still great and I play it sometimes. Sorry for half a post, kinda hit enter by accident. =(