or download cpu-z: http://www.cpuid.com/cpuz.php
you can mix ram, but it will run at the speed of the slowest stick. do not get ddr-400 since it can be problematic in PCs that don't explicitly support it.
I want to add a 1 GB memory module to my 256 MB Dell Dimension 2400. (WinXP Home, SP3). The machine supports DDR-266 (PC2100) and DDR-333 (PC2700). I don?t know which one is currently in it. If I add a DDR-333 module will it work OK if the original module is a DDR-266? If not, is there a way to find out what kind of memory it has?