NVidia just released the GT 440, which is a faster version of the GT240, but, contrary to the GT 440 they had for OEM's a few months ago, only has 96 shaders (exactly like the GT240), instead of 144.
Having said that, it has a higher clockspeed.
However, Techpowerup has tested the 1 GB GDDR5 of the GT 440 against the HD5670 512MB GDDR5, and the GT440 loses almost all of the benchmarks!http://www.techpowerup.com/reviews/ASUS/GeForce_GT_440/
So, assuming both scale down linearly with DDR3, I'd say the HD5670 is the best. And given the fact that the GT240 comes with an even lower clockspeed...
HD5670. Definitely the best card for the price.