I'm looking at some bikes that I'd describe as slightly off-road leaning hybrids, like the Gary Fisher Kaitai. The Kaitai uses a 700x38c. I don't want to get stuck on an overly oddball size, and looking about, I didn't find a huge selection of tires. If I can use wider or narrower tires, I'd have really good variety. How far in either direction can I go safely? Can I run a 700x28? A 700x42? Thanks in advance.