From what I've heard you're just always suppose to have matching chips.  Ive done something similar a long time ago though, a 256 and a 512.  It worked for a while for me - if I was you, Id call a local computer exchange store and see what they will give you for the 512 - you might be able to upgrade it to a 1g for pretty cheap.  Just make sure you bring your 1g chip with you when you go so you can make sure they match.

I am:  aka Velocity3d