I just upgraded my 512 mb iMac to 1.5 GB. Checked the system info, and it's up to 1.5 GB alright. But boot-up speeds, application speeds, and a few photoshop tests, seem to have no increase in performance whatsoever For example, with or without the extra 1 gb, Photoshop CS opens in ~11 seconds. I did a full radial blur on a picture, and BOTH memory configurations completed the task in ~5 seconds. Is something wrong with this picture here? Or am I simply not taxing the system enough to actually see a decent speed increase?