I think a 5670 is the most powerfull card you can get that ll run off the PCI slot only, without needing a external power plug as well.
So if you have a small psu, without access to plugs like those, its probably your best bet.
Make sure you got for one of the 1gb versions, with 1600mhz effective memory.
Like this one: ~69$ without MIR.
http://www.newegg.com/Product/Produc...-360-_-Product