You probably need to upgrade your videocard to be able to play Modern Warfare 2. 8500GT's are not known for their gaming prowess and you'd be better off finding a cheap 4850 or some nvidia equivalent like a 9800gtx.
you have a pretty decent video cafrd and your cpu is good and so is your ram but are you using vista or xp also try lowering your resolution that would probably help and when playing MW2 from what i've seen on gameplay footage you would want to try playing with no AA and maybe low-medium settings.
The graphics card will definitely struggle with Modern Warfare 2, especially if it already struggling so I would recommend an upgrade of this to get the best out of this game when it comes out. See the Graphics Card charts on this website and compare potential cards in your price range with your current card to see what sort of improvement you should get.
Just a quick note, Crossfire (and SLI) refer to having two graphics cards in a system. So in the one you referred to "ATI Radeon HD 4870 Crossfire", this is two Identical 4870s in the same system. I am from the UK so I am not too sure how prices compare here, but a single 4870 I would say is a very good investment. It has more than enough power for what you will need and should stay that way for at least a year certainly. So if you can find one of those at a good price, I would say get one of those.
Onto the CPU. I do think an upgrade (from a gaming perspective) may be imminent soon, but there is still life in your chip. It may begin to struggle on this game, so I would have a go at overclocking it. The graphics card upgrade is far more important than the CPU in your case, so with your budget I, personally, would recommend a 4870 and possibly a good after market cooler for your CPU and get that overclocked. If you can get it to reach about 2.7 / 2.8Ghz, you would be doing great, but even to about 2.5Ghz will give you a nice speed increase.
For example, I OCed my e7300 further from 2.83 to 2.92Ghz and saw about a 2/3 FPS increase on the same benchmark on Resident Evil 5, so every little helps!