Basically the GT 240. It's not particularly worth it to get a green version of a 9800GT. They do perform slower than a regular 9800GT. Also the difference between a GT 240-GDDR5 and 9800GT isn't that big anyway.
If you have the GDDR3 version then it may be worth the upgrade. Or if you have something like a GT 220, since you don't seem confident you know what you have, then the upgrade would be substantial. You need to find out this info. But if you have the GDDR5 version of the GT 240 then you should probably just wait until the next gen comes out.