If I was upgrading and wanted an AMD processor I would choose an AM2 motherboard because they are newer and likely to be about longer than the socket 939 ones. I mean look at this click here
Now if you have a 939 the prices have dropped dramatically click here about a year ago I bought the 3500 Athlon 64 retail processor and it cost me about £130 - now its £44.03. I do notice they are ou of stock at the moment but if you can get one at these prices it would be very good value. If you want dual core the AM2 chips are cheaper than the 939 ones which are getting harder to get anyway.