First question - does your motherboard support different size modules together - I would guess the answer would be yes, hopefully the days of matched memory are long gone :-)
Is this machine going to be your WHS box? If so, you can probably just get away with the 1 GB, otherwise I would try with both sticks in, if you think there is a performance hit, or you are starting to get freezes or crashes you didnt experience before, just pull out the 256 MB one.
Andrew