You are right there are only 3 slots.
So it seems that by buying one more 512mb would at least improve the memory and would be for the moment a less expensive upgrade answer.

What would be the specification needed for the 512mb? There seems to be a multitude of types to choose from.
I will look at the your idea to put in 2 x 1gb as well though, so could you tell me what the specification would be for those as well.
The reason I ask this is I have not the foggiest idea which to buy
I see all sorts of models and types, 128 pins and such mentioned but I do not know which type I need. Also 266MHZ 333MHZ. I seem to remember from somewhere that anything higher than 333 would not work on this motherboard. ( but I may be wrong )
Cheers.
Dave.