Sizing MySQL Database Servers for Drupal
I was waiting for more experienced people to answer, but one thing I will point out, since you're using innodb engine. You'll want sizeable amount of RAM to increase the innodb_buffer_pool_size so that it can hold your dataspace, if possible. So if your database takes up 6GB, it's feasible to set the buffer_pool_size to catch it in RAM (assuming you can address that much RAM...see below). But if the database takes up 64GB, it might be a stretch (at least for something like my small company's budget) to get a server with enough RAM.

You'll need to make sure your CPU architecture and OS is 64-bit. I ran into this recently, where an old xserve upgraded to Mac OSX 10.6 can't utilize >4GB of RAM per process. Probably not an issue with modern servers.