I was thinking of upgrading the memory in my computer, and was wondering where the best place (online) to buy it would be. I bought some years ago from a place that I can't find anymore. I don't want to get something cheap that won't work. Also, does anyone know if you can mix and match sizes of memory? If I have two 256 DDR RAM (total of 512) can I put in two 512's (in addition to the 256's)if there are four slots? It has to go in pairs. Oh and this is a Dell Dimension E510.