64-bit Toys for Athlon-64?
SUSE Linux Enterprise Server 8 for AMD64 has been around for a long time. That's why it was used in all of those Opteron benchmarks after it was released.
For something a little more affordable, SUSE 9.0 for AMD64 will be released tomorrow. (Along with the IA32 version I preordered)
For a no-cost alternative, you can download all 9 ISOs for the SUSE 8.2 for AMD64 beta here.