I don't think you can get a better or a nice, good PSU in 50$..... BTW I used my GT630 on a 350W PSU where it required 450W, (75W Power from PCI-E)... I've checked the ZOTAC GTX 750 1GB Card and a GPU which wants a 400W Power Supply WILL work on a good, branded PSU. Like you can run it on Cooler Master Thunder 350W... So, the final answer is that it will work on your PC but your PSU will run on a little heavy load and the temperatures of your CPU and GPU will be moderately higher due to that less 50W power supply.... Just check your PSU once before you buy that card and ensure that it's branded and of good quality and of above 350 Watts. Enjoy your new upgrade.
UPDATE: You can get any model of the GTX 750 Series, Even GTX 750 Ti's but just not the MSI GDDR5 Versions... They require 600W Power Supply. Check the model and power requirements once from the website or the dealer you are getting the card from. Good Luck.