In your case you'd benefit more if you got something like 6800GT (but the prices on them are more than $130, even if you sell 9700 it wont be enough).
You could wait to upgrade until Nforce 4 settles down and goes through some chipset revisions, pcie cards become more available and prices come down. Then in Spring when new cards come out you might be able to pick up 2 6800GT SLI for $250 a piece on that nforce 4 mobo and a cheaper A64.
It all depends how much are you in need of an upgrade. If it's for gaming and you feel your gaming is choppy, you can just get a new videocard. If it's for other work, and you need cpu speed, then sure A64 would be beneficial.
Right now A64 3200+ 90nm is about $180 and 939 mobo around $120. Those should overclock to 2.4 giving you roughly 3800+ performance. I guess the general rule of thumb is, if you are willing to wait (means it's not burning on a pan for you yet so you don't really need it yet), then wait as long as you can since prices will come down and performance will increase. I guess if you were asking this question, then you don't really need it just yet, since you would have already acquired it, no?