Can't POST, memory beep codes! HELP! (GA-7N400)

I'm putting together a computer using a Gigabyte GA-7N400 board with an
Athlon XP 2500+. I ordered 512MB of PC2700 for it but I can't get this thing
to boot up. It hasn't POSTed once. Each time I power on, after a few seconds
I get 1 semi-long beep which is repeated about every 3 seconds. From what
I've read this is supposed to be signaling a memory problem. So, I've tried
2 other 512MB sticks of PC2700 both from WORKING computers and still no go.
I also tried 2-256MB sticks of PC2100 and same thing. However, 1 of those
sticks doesn't cause any beep codes, it still doesn't POST, though. What do
you guys think? Have I been extremely unlucky and tried 5 pieces of ram that
are incompatible with this board or is it something different? Thanks in
advance for your help!

Mike