Increasing your memory will just increase how much data can be tossed around in the card itself. So remember, if the gpu clock isn't up to scratch, it won't be able to fill the bus lanes of the memory, which makes the memory overclock less effective.
This is why people usually push their ram along with their core because you want to unlock a few extra Gb/s in the bus lane data transfer, while you increasing the GPU's ability to gobble up that data. You can get away with not overclocking the ram very much, depends on the GPU. For example, the 8600gt's gpu really didn't use much data, yes, overclocking the ram unlocked it, but it wasn't held back very much by the slow ram. GPU overclocks were more effective.
Just be careful, overclocking the ram can lead to artifacts. I pushed my old ATI 1600XT to the limits, and I mean the limits. It then stopped rendering things properly, ran close to the temperature of fire, but I was getting awesome frames per second

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I suggest you just read up about overclocking. I haven't been in the game for a while, so just running off prior knowledge of my more 'rebellious' gaming years

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Ps, don't use the overclocks guys on forums get. They may very well be legit, buuut, they usually using expensive cooling, have a card from a 'good batch', and also have been overclocking and burning out pc components for years.