Well, DDR RAM is pretty easy to pick out. The designation is the theoretical transfer rate PC2100 can theoretically transfer data at 2133 Mb/s and PC1600 can transfer data at 1600 MB/s. There is PC2400 available from Mushkin I think and it has a higher potential clock speed. These correlate to the old RAM in this way: PC1600 is DDR of PC100, PC2100 is DDR of PC133 and PC2400 is DDR of PC150. They are supposed to be working on some new formats like PC2700 (PC166) and PC3200 (PC200). Crucial is a great way to go because the cost is low and the quality is very good. They offer 256 MB sticks of PC2100 at a really low price.