System instability for 1.6 GHz models

my 1.6 became unusable after the firmware update, always freezing after a few minutes. it appears that ram from different manufacturers and/or speeds will not work together after the firmware update. i pulled out the 2 x 128mb's (pc2700) that came with the computer and left the 2 x 512mb's (pc3200). it's working again, no freezeing. if you are buying ram for a 1.6ghz match manufacturers and speed (of ram)