Yes its possible, i own a amilo pi 1536. But i upgraded it with an Intel Core 2 Duo T7200 and Geforce 9600m GT. Visit this site for more information because replacing the GFcard takes a few stepps.
Better go for ATI. Less problems on boot, less heat, less power consumption. Go for ATI HD3650, HD3670, HD4570, HD4650, HD4670. Don't buy them with Acer Video Bios cause it won't boot.