Your first option would not get you the performance you want in crysis unless you are gaming at 1280*720 or 1280*1024. Going from a 4850 to a 4890 isn't much of a deal unless you can sell your 4850 for a decent price.
Your second option, go for another 4850 in crossfire if you do that. But the processor would be overpowered for the GPU.
1. The best bang for your buck would be to add another 4850 in crossfire on your current board ($130), if it supports it. In this case, you might be able to squeak by by overclocking your CPU to 3+GHz. The OCZ Vendetta II is a good heatsink. Or if you prefer to get a new CPU, you could go for a core 2 quad. If your board doesn't support crossfire you could get a new one for under $100 that does, keeping socket 775 and your current processor (or the core 2 quad). This would perform fairly well but might not do as well as you want depending on your screen resolution and what you consider acceptable framerates. Crysis is very demanding.
2. If you have the money, by all means go i5. I would go for 2 radeon 5850s in crossfire, depending on your budget. If you don't want to spend that much on graphics cards, there is no point going i5 though. If you want to cut your budget, you could overclock a phenom II 955 on an am3 board (DDR3 ram) with the same graphics cards. These setups would be able to play crysis on very high with a little AA at 1920*1200 or 1080.