Be sure to have HW Shading disabled in the Preview Render options.
It not only slows the OpenGL preview down, but also makes the mouse behave more reliable.

With your hardware configuration you should be able to have 4-5 hires figures with clothing and hair items in the scene without too much slowdown.

Above that you might run out of memory and get into memoryswapping and that will slow things down considerably. (You can see this in taskmanager in Windows)

Be also aware that not only hires figures are memory intensive, but also some clothing items, dynamic hair and props

I have scenes with more than 10 hires figures (V3,SP3,A3,M3) with clothing and hair and props which is still manageable on a XP64. Intel Dual Core, 4GB, Nvidia 800XL and which will render 4000x3000 images. On an XP32, AMD 6300, 2GB, ATI video these scenes are extremely slow in the preview mode.