I'd be wary of changing PSU, for the reason unclene gives. 230 watts is fine for a faster motherboard, but not if you fit a fast graphics card. Many motherboards identify the power they require - add the graphics card's consumption and you have it.
Is the case a "standard" one? My daughter was given a PC with a slightly different layout, designed for one particular manufacturer. It went in the skip.
My laptop is 1.1GHz and does a fine job. Speed is not a problem. I just had to clear out a lot of the junk that ran in the background and slowed it down. It might be worth doing a Ctrl-Alt-Del and looking at the Task Manager to see what's running.
My old 600MHz machine sits here as a fileserver, and does many things as fast as the desktop (3.2GHz). It runs Linux, which doesn't have all the M$ baggage.