I think he is trying to say how much should he get before the cpu gets no benefit from the extra memory.
Well i would say the more the better (within reason) i would aim for 512mb, that would give you a quick boost (thats if your board can take it)!
Im guessing your using sdram which is very expensive as not many people make it in volume anymore.
I would shop around and consider e-Bay as some of the modules i looked @ where well over £100, you have to ask your self is it worth it to save 15 seconds loading something!
If your lucky enough to have DDR memory modules, they are cheep as chips at the min for budget lines and if you pay a tad extra for some high speed mem you can always put them in a new rig when the time comes!
If you want to put more than 1gb in it wont hurt, i just dont think you will get all that much from it! It would help with the video side of it a tad but thats all! It all depends on what you can afford.
thats just my opinion!
Hope that helps, i wont reply again to this, as im going on my hols ( YAY )