Pendragon2U,
Some things to consider:
I do quite abit of surfing for motherboard/support problems, and as I have the Pro Gigabyte board, I see quite a few post about it.
The Problem that you describe has been seen before and turned out to be a memory related problem. The gigabyte Nforce2 boards are very picky about ram, so i would have to ask you if you have run memtest and if that 512 is two sticks of ram, have you tried with just one.
The other thing you need to know is that the strength of your power supply will play a significant part, so what are the 12v, 5v, and 3v outputs...
Here is a link for you to a gigabyte
New Owners Guide Lotta good info in there.
Also there is a very good Gigabyte specific forum where you can get lots of good help
Gigabyte Forum You tell em MOM sent you and they will take good care of you
Laterz,
dafanman