What PSU do I need?

The 660 ti only requires a psu with 450W and with 24A on its +12v rail(also two 6 pin pcie connectors), so any quality psu within ~500w will do, the xfx 550 core edition is cheap there so you might get that psu, it's a quality one.

Its most likely going to be a standard ATX format PSU, just make sure that is what fits in your case. As for power requirements, l would go to http://www.extreme.outervision.com/psucalculatorlite.js...
and fill in the page with your hardware and add in whatever accessories that will be commonly connected to your machine. The results will be what wattage PSU you need. buy from a good dealer, and buy a good brand, like Antec.
Best of luck.

Thanks to both of you, you were very helpful. If I don't find that XFX 550 I'll go for an Antec ~500w.
Sorry for my english, I'm still learning hehe.
Thanks for your fast answers  

There's others, and it depends on the antec model, usually the brands to look out for is the seasonic, xfx, rosewill capstone, corsair tx v2, hx, antec neo eco.
Usually in the usa, the better deals are the xfx, seasonic and capstone psus.