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) "Money can't buy you happiness." Maybe so, but it can make your unhappiness a LOT more comfortable!