depending on how old it is, you should easily be able to run a 5670 on your psu with no problems. a lot of people do it, it's basically the reason for the 5670's existence, its the most possible graphics power you can get from a generic 300w psu.
you can get 5670's for about 90 ish dollars on newegg.
another route to take is to spend around $30 on a 450w psu, and then a $100 on a 4850. that'll give you a lot more performance than a 5670 can offer