I'd avoid a guid if at all possible.
Make sure you have an index keyed first on c1, so that determining the max(c1) is a single, quick seek. An INSERT of a single row shouldn't take too long, so even if it's briefly blocked, I wouldn't expect too much of an issue overall unless you're issuing an extremely high level of INSERTs.
SQL DBA,SQL Server MVP(07, 08, 09) "It's a dog-eat-dog world, and I'm wearing Milk-Bone underwear." "Norm", on "Cheers". Also from "Cheers", from "Carla": "You need to know 3 things about Tortelli men: Tortelli men draw women like flies; Tortelli men treat women like flies; Tortelli men's brains are in their flies".