If you need to sum the entire Transactions table 300 times a second (with no filtering or grouping), I'd say you need to either reconsider your design or your requirements. One (or both) doesn't make sense.
No, functions like SUM do not cause lots of deadlocks.
Gail Shaw
Microsoft Certified Master: SQL Server, MVP, M.Sc (Comp Sci)
SQL In The Wild: Discussions on DB performance with occasional diversions into recoverability