I would not worry about that too much.
However I will make sure any parts I recommend use less than 150w so your total system power consumption remains less than 450w, giving you a lot of room on that generic PSU.
So I will remove my recommendation of buying the AMD 7870 and R9 270X. The older AMD 6870 and GTX 560 also use slightly too much power for me to recommend if you are worried about the generic PSU.
I am also going to remove the R9 270 from my recommendation as you are on a tight budget.
My belief is the GTX 750ti is the most powerful graphics card I can recommend for you and you can also feel safe knowing it uses so little power that you should not have any problems with your PSU.