best videocard without an extra power connector would be a hd7750, stronger than both.
I went to a computing shop and told the vendor that I wanted to order an AMD Radeon HD 7750 and then he told me that my 300W PSU won't support it, and it costs 83€, can't afford it.
yeah the recommended wattage is higher, but there are many people who run this card on such a psu.
and your gt620 has a power usage of 49w, the hd7750 got 55w, your psu should be fine