The nVidia GT240 is a little better than the HD 5570.
I would by a 512MB version of either card. There is a 512M and 1GB version of the much faster HD 5670 and there is no performance difference between the two version unless you like looking at small differences in benchmarks; I am talking about 0.0% to 0.3% difference.
I've noticed that in many tests the 5570 is comparable in performance to a vanilla GTS 450, so that's also a consideration. All of the above are good, solid cards with nice low power consumption, so you don't really lose with any of them! Just go for whichever is cheapest.
make sure the version of the GT240 is the ddr5 version and NOT the ddr3 version.
if you have the correct ddr5 version then I would get the GT240.
GT240 is directx10.1 while the HD5570 is directx11 - no big deal yet.
I agree with - jaguarskx. if you can get the HD5670 and be done with it..