Also make sure ghost mode is enabled in the master page.
3 or 4 is really low for your spec...I think I used to manage about a dozen or so on XP32.
But to get to 40 you really need a 64bit os to beat the single program ram limit. Most people can only use 1.7Gb per program (think for some its only 1.3Gb) in a 32bit o/s & your DAW already uses a chunk of that & if you also use any sampled instruments that load into memory you'll quickly run out. Check your task managager performance tab while running a project to see how much you are using...if its 1.3Gb or 1.7 when you hit problems then that is what you need to address....free up some memory.