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) A socialist is someone who will give you the shirt off *someone else's* back.