[Build Help] What PSU should I put in my Inspiron 530?

I want to upgrade my Inspiron 530 with a Geforce GT240 but when I installed the card it wouldn't run on the computer past safe mode. I concluded we needed more power to it. The minimum amount of power recommended for the card is the amount my computer has (300w). So my question is: do I need a larger PSU and which one is over 300w, affordable and can be put in my Inspiron 530? Help would be very appreciated :)