I started out with a Topeak Explorer rack and a RX Trunkbag DXP. The bag is ok for when I'm only working a half day, but is too small to squeeze in a lunch. I just bought these:
Amazon.com : Bushwhacker Moab Black - Bicycle Rear / Front Pannier Cycling Rack Pack Bike Bag - w/ Reflective Trim - Sold as Pair : Bike Panniers And Rack Trunks : Sports & Outdoors
They came yesterday while I was at work, so I haven't tried them out yet. I did put them on Tilly this morning, and they are nice and roomy, and I can still use the trunkbag with them. They aren't waterproof, but I figure I can just pack things in garbage bags if it looks like it's going to rain. They have good reviews on Amazon, and for the price I couldn't pass them up.