jleethal :
I have a 400 watt PSU. I'm willing to spend 300-400'sh dollars.
Here is what EVGA recommends for their graphics cards:
GTX550ti needs 400w with 24a on the 12v rails plus one 6-pin PCI-E power lead.
GTX560 needs 450w with 24a on the 12v rails plus two 6-pin PCI-E power leads.
GTX560Ti needs 500w with 30a on the 12v rails plus two 6-pin PCI-E power leads.
GTX570 needs 550w with 38a on the 12v rails plus two 6-pin PCI-E power leads.
GTX580 needs 600w with 42a on the 12v rails plus one 6-pin and one 8-pin PCI-E power lead.
GTX590 needs 700w with 50a on the 12v rails plus two 8-pin PCI-E power leads or 4 6-pin power leads.
With your current psu, you are looking at no more than a GTX550ti.
Here is a good Antec HCG 620w psu for $50 after rebate. It would run up to a GTX580.
http://www.newegg.com/Product/Product.aspx?Item=N82E16817371048
You can get a GTX560ti superclocked for $220 after rebate:
http://www.newegg.com/Product/Product.aspx?Item=N82E16814130610
Perhaps even a GTX570 is possible.