Having more RAM on a 32-bit OS (like 2.5+) will actually make your pc slower, because it takes more time to access the large amount of memory. If you
did however want to see a change in performance, check the L2 Cache size on your processor. L2 Cache is like RAM, but is accessed more quickly. Average size is 2MB, although most processors are expanding to 4-6MB. I know Macs have 6MB, which is really fast, but then again OS X comes in 64-Bit on new Macs, so you could easily upgrade RAM... But no one here has a Mac, right?