Greetings!
It may very well be the PSU. The GT630 doesn't require any additional power (no 6 or 8 pin connector) and draws its power through the PCIe slot on the motherboard, but depending on how much power the rest of your system is requiring, this may be an issue. With a 300w power supply, I am making an assumption (oh no!) that you are using a prebuilt system like a Dell or HP? Unless these systems came with a dedicated GPU originally, they may be under-powered to support the addition of one. 300w is a pretty scant PSU and depending on the manufacturer, may not even be able to deliver that much...sufficient for the base system requirements and nothing more. Typical for OEM builds.
I would start by borrowing a friend's PSU (that is of higher wattage - ~450 - 500 watts) for a couple hours if possible and plug it into your system to test if it all boots up ok. This is a totally free and easy way to determine if your power supply is the culprit. Another way is if you had a meter that would allow you to test power draw of the system from the wall plug without the GPU installed...If it is greater than 200 watts, it could be the issue for the GT630 requires a 75 watt provision, and if your PSU is weaker than advertised, well, there just ain't enough juice.
Hope this helps!