I'm not sure how the GT640 and GT730 compare for performance, but the GT730 would use less power. A GTX750Ti is listed as working with most modern 300W Power Supplies so the GT730 in particular I would guess at no more than 40Watts.
*I couldn't find any benchmarks for the GT730 but I did find a couple videos to get a rough estimate of performance. For $70 it's a really nice card which...
for the small form factor there is a 275 psu which should handle the weak 6570 without problems.
Also there is room for 1 low profile gpu according to specs.
Make sure you don't have the Ultra small form factor one because that one doesn't have a pci-e slot for a GPU and the PSU is too weak to handle it anyway.
The HD6570 is fine for normal tasks, and very light gaming so if you're aware of all that chances are you'll be okay.
If you want better gaming there may be better suited cards like some of the 750/750ti low profile cards. If they fit as some use two slots. They also cost a lot more though..
I'm not sure how the GT640 and GT730 compare for performance, but the GT730 would use less power. A GTX750Ti is listed as working with most modern 300W Power Supplies so the GT730 in particular I would guess at no more than 40Watts.
*I couldn't find any benchmarks for the GT730 but I did find a couple videos to get a rough estimate of performance. For $70 it's a really nice card which again should have no power issues:
Watch Dogs https://www.youtube.com/watch?v=Z_7tegJ9apI
**FYI, you should not be playing the above games at 1920x1080 with this card. It should be either 1366x768 or 1600x900 with some anti-aliasing and a good balance of the other quality settings to achieve your performance goal (even try using the "GEFORCE EXPERIENCE" default setting which aims for 40FPS with VSYNC OFF by balancing the various settings to give you the optimal result in their opinion.)