if u can get a cheap set.. going with whichever optioin shouldn't be too much of a deal.. jus that with the 1 stick of 512.. u can upgrade later on for more ram if u choose to do so..
and usually the 1x 512mb is acutally cheaper (at least on newegg) when you include shipping, so i would go with the 512mb. Or you can get 2 of them to replace the 2x 256's that u have now.
You could have compatability issues... Different speeds of RAM will cause the faster RAM to reduce its speed down to the slower one (if any), and it needs to be the same make and model usaully for them to operate properly...
I recommend getting a total new set of RAM, just 2 x 512 MB RAM, and taking out the current RAM you have in.
i had the exact same situation but i went with 2 more 256 and got each for around 20 dollars, great deal from newegg. Wasnt exactly name brand but has worked perefect so far.