SW benefits little with increased graphics card memory beyond 128 meg. I doubt you'd see any difference between 256 and 512 at all, unless you have an insane number of windows open concurrently.
Given that amount of ram useage, I'm assuming that you are using the 64 bit SW? If you are not, thats likely your current limiting factor.
If you are working 64 bit, I'd look to increase system ram. Windows has a nasty habit of paging to disk, even when there is quite a bit of ram space available. Next time you have the big assy open, look at task manager. If 'physical memory total' - 'physical memory available' is a larger value that what your current commit charge is, then you are paging to disk. I've seen windows begin paging when 30% of physical ram is still available.