Depeding on what you can get the second 275 for a new MB for what you have is about $93;
ASUS M4N75TD AM3 NVIDIA nForce 750a SLI ATX AMD Motherboardhttp://www.newegg.com/Product/Product.aspx?Item=N82E16813131637
This might be the cheapest way for you to go and you can keep your power supply and use the 275 you have in sli. When the decision is to be made , you are the one paying and it's going to be what you want to do. It is also going to depend on your budget. We can only give you suggestions and different setups.