ram support is incoming from your memory controller.
which is in your CPU in all amd systems.
ddr2 is supported.
abit website specifies 800mhz to be the best frequency supported.
so the bios settings shouldn't exceed 800mhz.
amd's website specifies in the general AM2 spec pdf 400mhz (ddr2 800)
http://www.amd.com/us-en/assets/content_type/white_papers_and_tech_docs/43042.pdf
if you get 1066 mhz ram, set the bios default to 800. also, your system could end up being unable to boot with it.
your best choice, 800mhz with a lower latency.
1066mhz will only be handled by the next generation motherboards.
but by then, integrated controllers should support ddr2 and ddr3 but on another, more versatile socket. integrated mem controller was good on S939 while it limits now as memory is upgrading faster.