There is about a 6% - 8% difference, I have read, between equal amounts of matched and non-matched RAM. Having more total RAM however, is almost always better.
If you can afford 2 Gb matched (2 x 1 Gb) then that is optimum. But you'll probably get more benefit from the extra 768 Mb than from the Dual Channel.
Thanks
Trevor
CanadaRAM.com