I don't think 1 billion transactions per day is that scary any more but then it depends what you mean by transaction. A transaction can be a simple record insert or a batch of records inserted as a single transaction.

It also depends on the read/write mix of traffic to the server, page life expectancy, buffer cache hit ratio etc.

One of our lines of business pushes 100 million records per day precisely how that breaks down to transactions is hard to say but we have another 48 lines of business and running a mix of versions of SQL Server from 2000 onwards and they certainly aren't on high end boxes.