If your mobo can take it, some of the newer Phenom II x4's are nice too. For the same price as the 9550 you can find a comperable AMD. It may not be as good as the i7, but the top AMD's can often beat the 9550 or 9650 and cost a bit less.

If your mobo can handle it, the cheap option is a new quad core CPU (~$200) and 8GB of RAM (~$100). I'd start with those and then look a new video card after that - get something with at least 1GB of video memory. The 4850 or 4870 is a nice performace/price point right now.