All PSUs gradually deteriorate as a result of ageing capacitors. 280 Watts is not much to begin with, and adding a graphics card may not be the ideal thing to do.
About adding RAM - 533 MHz RAM is hard to find, and you may have to go with 667 MHz RAM (I have done this, and it worked ok). Slight improvement in performance. Install the 667 RAM in slots 1 & 2, the 533 in slots 3 & 4. RAM does not use much power, but graphics cards do.