The ATI equivalent would be the ATI HD5670 1GB PCIe and looks to be much superior to the GT240 for the same power/heat usage. (Forget the GT240)
GeForce GT 240 GDDR5 vs Radeon HD 5670
The GeForce GT 240 GDDR5 has a GPU core speed of 550 MHz, and the 512 MB of GDDR5 RAM is set to run at 425 MHz through a 128-bit bus. It also features 96 SPUs, 32 Texture Address Units, and 8 ROPs.
Compare those specs to the Radeon HD 5670, which comes with a clock frequency of 775 MHz and a GDDR5 memory frequency of 1000 MHz. It also makes use of a 128-bit memory bus, and makes use of a 40 nm design. It is comprised of 400(80x5) SPUs, 20 Texture Address Units, and 8 ROPs.