I have read up about RAM, and am going to buy some more for my computer (currently running 1gig ddr2 533) but I am not sure of the speed differences between 533 and 667. Is it really big, or barely noticable? And which would be better - one gig of 667 or 2 of 533 (I am on a limited budget, so I can only afford one or the other)? Thanks very much.