SQL Server 2008 R2 with huge amount of transactions
It is difficult to estimate server load based on "number of transactions" because cost of a transaction can vary greatly depending on the design of your DB and the nature of the query.  Enterprise and Datacenter editions are your versions to consider for situations with high volume and high scalability. Microsoft has comparison charts showing what the processor/memory limitations are for each, extra features, and virtualization restrictions. (Search: SQL Server version comparison)

What type of application is this supporting and for what business type?  Will it be performing strictly transactional OLTP tasks or are you looking to support reporting and analysis with (a separate instance of) the same product?