Your power supply, according to Dell, is a gold certified 250w unit. Dell also says you can use the 7570 as an optional gpu. The HD7750 is also a good choice - it is the fastest gpu you can get that operates on the power provided by your PCIe slot alone.
Regarding replacing your psu, Dell has a habit of using parts that are "non-standard" form factors, especially in slim line cases. Your best bet is to measure your psu, including the spacing on the mounting screws, and see if it is a standard ATX form factor. If not, it will be difficult to find a suitable replacement unless it is from Dell.