For long-term storage and delivery of all our downloads we use Amazon S3, and one can hardly find any more robust and high-performance a service than that within the realm of affordability (doing any better would go deep into "if you care what it costs, you can't afford it" territory).
Aside from downloads, our main order-handling servers typically run at a mere fraction of their maximum capacity while processing orders in the millions of dollars every month. We have yet to encounter any problems even with some of our sellers getting nationwide network TV publicity, so we wouldn't anticipate any problems handling any expected volume of orders and downloads.
