Having enough ram to not have page-outs to your hard disk will drastically improve your performance. Gaining dual channel would have a minimal performance increase. I'm running 3 gigs in my uMBP right now until the 4gb chips drop below the $200 mark. And then, I'll have 6gb total, which traditionally wouldn't have dual channel.
However, I also remember about 9 months ago someone linking to whitepapers for upcoming (now available) intel chipsets which could retain dual channel with mismatched ram sizes. Perhaps the nVidia Ion platform incorporates this? Either way, more ram for me trumps dual channel.