You can try giving windows a bigger page file for memory since you eating so much of it.
right click my computer select properties, click the advanced tab then performance (settings) then the advanced tab in that, select virtual memory change, select custom size for your OS drive and try setting the initial size to 8000MB and the maximum size to 16000MB, click the set button and reboot (sometimes the system can get unstable for a second so have your programs closed) then click ok and reboot. it will take a while for the changes to take effect but I think it will help you.
With all that ram being filled, are you sure you scratch disk is operating and setup properly?
EDIT, you could also upgrade to Vista 64 bit, it has much better memory management, but if your programs don't work in Vista that could be very expensive.