if you're running at a solid 3.3ghz, wouldn't be worth it in my opinion, unless you're also ready to replace your 6800 with a pci-e.
i'd doubt you would notice any diff unless you run your games at lower resolutions (but who spends $400 on a video card to do that??) or benchmark.
the athlon may be a bit slower in most encoding tasks, but as with the previous scenario, i'd doubt you'd notice any difference unless you actually benchmarked.
if you're used to running multiple cpu dependant tasks you may even feel it's a downgrade at times.
personally i'd way till the newer revision cores come out; the upcoming E0 core stepping, which is due early next year, is rumored to support additional features SSE3 instructions (prescott supported it from the beginning), as well as 4 write combining buffers, and even lower power consumption than the current 90nm a64 cores, or even until dual cores come out later in the year.