I had 700x28s on my CX bike most of the time I had it. They worked on a gravel rails-to-trails in the mountains around here. A few times they were a bit dicey in deep gravel, but even then they managed to keep me upright. These days I do some light gravel on 700x23s.
If you don't go wider than 25 mm, try GP4000s, but, if you do, get them from a shop in England. The LBSes around Seattle charge $75 per tire, while Ribble and PBK tend to charge more like $35.