At the moment I have a 512 PC133 SDRAM and a 256 PC 100 SDRAM on my board.I'm thinking of upgrading (always have this 'not fast enough' feelings !).Is there any after effect when a PC 100 and a PC 133 work together ? Thinking of getting another 512 stick a good idea ? My PC is running Celeron 1.1 Ghz with 20 GB HDD.Any opinions will help.Thanks