I have 8GB and my 5 wow clients and the OS uses ~6gb. So yes getting more ram is generally helpful. I would prioritize the order of upgrades to be ->
Ram
HDD -> SSD
CPU
GPU
My 4850's are only using about 60% of capacity. This could mean I have a bottleneck somewhere before the data gets to the GPU. It may also mean that I set all the catalyst drivers to performance over quality.
Drommon
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