W/o knowing any thing more about your system, this is just a guess. But the MSI N750ti-2GD5TLP is the fastest single slot card I know of. However, it is 2.72" wide, so best to measure. A high quality 300W PSU will be fine. Anything less than that and you will be asking for problems. http://www.newegg.com/Product/Product.aspx?Item=N82E16814127836
W/o knowing any thing more about your system, this is just a guess. But the MSI N750ti-2GD5TLP is the fastest single slot card I know of. However, it is 2.72" wide, so best to measure. A high quality 300W PSU will be fine. Anything less than that and you will be asking for problems. http://www.newegg.com/Product/Product.aspx?Item=N82E16814127836