If using Windows, get a second 4gb stick...4gb is enough for linux, barely cuts it for win.
Really don't get this memory thing. I have eight rigs, each with four cards, all running (linux) with 1GB. Vertminer never uses over 250mb, including caches. Adding unused ram isn't going to do anything.
In windows the amd libraries compile the opencl file for the cards as one big (memory)image and upload that to the cards. If you don't have enough memory one or more cards won't start. Once the miner is running it uses hardly any memory at all.