That's not the best use of your money and time. You'll spend up to a few hundred dollars a month (or <$80/month if you constantly shut down the instances and only use it a few hours a day). You're better off getting a reserved instance, which will save you at least 40-60%, based on the instance type. On the higher end of things, you're definitely better off with a dedicated server.
Try out ec2 whilst keeping the desktop for a few weeks. If it goes well then you will earn some cash from the desktop sale along with learning how AWS works. In the long-term however, I'm not sure if it is the most economical option -- that really depends on how much power you need along with other factors.
Is there really sufficient extra value in doing this? EC2 is 'cheap', but surely not cheaper than an already-paid-for box. For development work, do you really need the flexibility of more CPU/storage/IO etc? Or are you simply in the mood to have something new to play with?