The CPU is the expensive part so lets try and get a mobo that can use your old CPU.
Important question: Are you using the onboard graphics, or an AGP graphics card? You will be hard pressed to find a replacement motherboard with AGP on it. It's all PCI-E these days. If you buy a motherboard with PCI-E and you have an AGP graphics card, you will need to replace the graphics card or settle for the onboard one.
Here's what you need to make sure remains compatible between your new motherboard and your old one:
1) Support for your CPU (Athlon 64)
2) Support for your RAM (400mhz DDR)
3) Support for hard drives (SATA, if you use it)
4) Support for your Graphics Card (AGP, if you use it)
Athlon 64 CPUs will work in a Socket 754, 939, 940 or AM2 socket design. AM2 is the popular one right now so I would go for one of those.
Most of these will work for you:
http://www.tigerdirect.com/applications/category/category_slc.asp?CatId=2320&name=Socket-AM2-Motherboard&I personally like this one:
http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=4364483&CatId=2320
