if you are in US, Radeon 270X = $200, GTX 760 = $230
both are the most popular gpu paired with fx 6300
between your options, i vote gtx 750 ti because of its newer gen, less power. the difference in FPS isnt noticeable
edit : you cant run gtx 660 with 420w PSU, afaik, that psu isnt certified (so that it isnt true 420W)
only 750 ti can run on low power psu like that