As other commenters have pointed out, 500 is way more international than YC. Over half of the companies in the current batch (of which I am one) are international. Dave McClure, Paul Singh, etc spend HUGE amounts of time travelling the world looking for the best trends and startups. In that sense, they select from a much wider field of applicants.
500 itself has no problem giving money to international companies - however, most companies flip-up or establish in the US to make it easier for other investors. It does depend on the company though. Some companies come to the US to enhance their network, not necessarily to crack into the US market, so setting up shop here legally doesn't make sense.
For visas - they provide references, etc that are needed and are very quick and responsive about it. That said, they will not "sponsor" you to come in on an H1-B or anything like that.
There have been many international companies that have gone through the accelerator program, including ones from Asia (Hong Kong, Taiwan, Japan, India), South America (several from Brazil) and Europe (Croatia, Estonia, etc).
It's a great way for international companies to quickly get integrated into Silicon Valley.
