I would simply go with a new build. You may or may not be wasting money with the e5700. But if it is not officially listed as a supported CPU for your mobo, then most likely you will be wasting your money.
While AMD CPUs may be less expensive than Intel CPUs, they are also less powerful. This can mean that you would upgrade again sooner rather than later which means spending more money in the long run. AMD's FX series also tends to consume a lot of power compared to Intel's CPUs; about 60w - 80w more. An Intel Ivy Bridge CPU is roughly 30% more powerful on average than an AMD FX CPU assuming the same clock speed.
How much more money that will cost you on your electrical bill depends on your usage and how much you pay per KWH. If you play games 15 hours per week every week and pay $0.10 per KWH, that works out to a little over $6 per year or $0.50 per month. If you live in a city where electricity is expensive (like NYC), then double that. Small, but over time it could add up which can make up a good decent amount of the price difference between an AMD CPU and an Intel CPU.