well I suppose adding a 512MB stick would be alright. though sometimes having all 3 RAM slots filled up can cause a few bugs, though that is motherboard dependant
well like I said, it's motherboard dependant.
it may turn out fine, it may not. though usually if there is a problem, it is not severe.
some motherboards just make the RAM run slower, some motherboards will be unstable, some don't have problems.
Okay, I appreciate your help and I'm off to the tech store now, for real this time , I'll try to work something out with the salesmen there, agree for a return guarantee or something, have a good day!
you could try getting one 512MB stick now, and if you find there are any problems having all 3 slots filled, you can go up later and get another 512MB stick, and replace the 2x256MB sticks.