Looks like you have 512 meg stick mixed with a 256 meg stick. Try just the 512. If you have 3 256 sticks, try just 2. If this doesn't work, try a bare post, with just one stick of memory, cpu/heatsink, and video on a phonebook or other non conductive surface. Be sure your case speaker lead is connected properly for beep codes.
Im not an AMD guy but i seem to remember something along the lines that a A64 requires PC3200, so if you are trying to use PC2700 the BIOS may have a problem starting up with this memory thinking you OCed too far, this is just a theory, someone else with more experiance with AMD CPU's jump in here, as i have little to no experiance with AMD machines, and dont know how they would react in this situation.