Wes Barron.....If I understand your question ......you currently have a pc with a stick of Kingston 512 mb PC3200 , DDR400 ...184 pin unbuffered , non ecc Ram....
The new ASUS P4C800E - motherboard according to the detailed specs will support up to 4gb total ram.....
184 pin DDR PC3200 unbuffered and either ECC or non ECC Ram...... So based on what you say and what I read in the detailed board specs you can use the ram you have.
If you have anymore questions , don't hesitate to ask.
Hope this helps
dl65