before I had 1GB I had 512 and I had a 128 stick of generic pc ram in the mix. it worked totally fine but it slowed down my xbench memory scores about 20%. remember that all the ram in a system can only run as fast as the slowest dimm.

check out this link for more specifics:

http://www.macgurus.com/products/motherboards/mbppcg3bw.php
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Mac mini C2D 1.83GHz - 2GB RAM - 200GB 7200rpm HD
Sawtooth w/G4 1GHz - 1GB RAM - SIIG SATA w/2x1TB HD
B&W w/G4 600MHz - 1GB RAM - 2x80GB HD
Sawtooth G4 400MHz - 512MB RAM - 40GB HD
Dell Ultrasharp 2007WFP 20" Widescreen LCD
2.94TB Total Storage