I'd go for one of the cheaper A64's and upgrade to a faster one when it drops in price...
Personally just got myself an A64 3k+ with an MSI K8N Neo Platinum and so far I like it.
Got and increase of ~5k points in 3dmark2001SE (and 300 points in 3dmark03), which isn't too bad considering I kept my old ram and videocard.
By choosing a 754 cpu, you'll be getting the cheapest A64's, but also the option of upgrading to the 3700+ when it drops in price.
If you go for the 939, you'll have to pay more for the cpu (as they are faster than 754's), but it can last you longer. (How long, I don't know, since it'll (most likely) only last until AMD increases the HT speed again.)