Help needed with memory problem

Hi,

I have just installed win 7[64 bit] on an SSD. I aim to increase the amount of memory in my system as 4GB is not enough.

I have an Asus P7P55D motherboard which will accept 4Gb modules[x4].

Unfortunately one of the slots is damaged so I will be only able to install 3 memory modules.

Will I notice much difference in performance by just installing 2x4Gb and using one existing 2Gb module? Or should I use 3x 4Gb modules? I use my system mainly for photo editing.

Many thanks

Peter