t1.micro is fine if your site fits into the limited memory and gets practically no traffic.
m1.small is reasonable for sites that get steady but low volume (for some definition of "low").
c1.medium is the next step up and feels pretty zippy if you've been using m1.small for a while, as it gives you 5X the CPU for 2X the cost.
The cool thing about Amazon EC2 is that you don't have to commit long term to any given instance type. You can start it up, test out the site, and then move to a larger or smaller instance whenever and as often as you want.
Here's an article I wrote about how to switch an instance to a different instance type:
Moving an EC2 Instance to a Larger Size
Note that to go above c1.medium into the m1.large and above, you'll need to switch from 32-bit to 64-bit architecture so make sure you document the exact steps you used to install and configure your software on the instance as you'll need to do it again when you switch architectures.
Note: My experience is entirely with Linux on EC2. I don't use Windows and don't know if it adds any extra performance overhead, but the core point is still valid about trying it out and seeing what works for you.