There's no way I would recommend a processor upgrade from a 4000+ to anything right now. As others have said, wait for the AM2 to debut and see what happens to prices as well as the newer benchmarks.
Wusy has asked the right question, where do you want your speed? Gaming? If that is the case first ensure you have a good setup in other areas such as: PCIe SLI Video card setup? 10K RPM Sata drives in Raid 0? (at least for the O/S and game programs since loading of textures from the hard drives will go faster as will the loading of initial environment details.) Are you running dual channel DDR? Believe it or not some folks have Dual channel boards with single dimms.
I like to break my machines up in sections and look at how I could increase them. Usually like this:
CPU/Mboard - since they go hand in hand.
Ram
Hard Drive
Video system
Audio system
Now if you make a small spreadsheet with what you have in it, and what you CAN do to increase performance in the others it will eventually lead you to a pretty good decision point.
And if you need more information, post your setup, I'd be happy to make recommendations or give opinion on how your current hardware could be enhanced.