1 512mb, 2 128mb and 1 256
In three slots? Do you mean you have 512 in one slot and have three more spare? No you can get any size, you want to have all the same speeds though. If you buy one slower than the rest, they will all become that slow. If you get faster ones that the one you have they will all be wasted and go as slow as your current one.
If you have a 512, I would go for another 512... U2U