Mixing RAM memory?

I have a desktop with three 164-pin memory slots (yea, old shit i know) with one 512MB stick on it, and I've been playing games for a while with this royal turd.  I'm getting sick of this, so I'm thinking about getting some dirt-cheap memory off Ebay and sticking it in along with my 512MB.  The model's will not match, and I'm planning to fit all three slots with two 256MB, and my current 512MB.  Will this work or should I just kill myself for my computer failure?

You only need to balance your memory modules if you have an even number of slots, and then only occasionally.  Make sure you get teh same or higher speed modules though.