If you are migrating to AWS I take it that your existing offering is either on your own physical tin or virtualised in your own data centre?

I'd seriously recommend that you engage one of AWS's own architects to help with this. They are a talented bunch but be aware that they are priced to make sure that you focus on using them for the important stuff.

You may also have to consider re-designing your DB and application to break it down into smaller units. There used to be a joke going around that AWS provided a hyper-mega-ultra-super-DB server or as DBAs called it "small".

It's not quite that bad now with the R3.8xLarge being 32 vCPU, 244GB RAM and a 10Gbit network connection.

You can request provisioned IO which I think goes up to 10,000IOPS.

Make good use of http://calculator.s3.amazonaws.com/index.html Forget copying/pasting prices into Excel you will miss something and if you don't your spreadsheet will become a maintenance nightmare.