If you do it right you won't see any effect on your PR at all.
Yep, what oilman said.
UNLESS your hosting company has a link to your site from a good PR page and your PR is reliant on this link (ie, you don't have a lot/any other external backlinks). I took a wee nosedive when shifting hosts for this reason, but really it was the kick in the pants I needed to get out and get some more valuable links.
traffic is WAY WAY DOWN
Your hosting company, assuming the server is solid and has no significant downtime, won't have an effect on how much traffic your site gets... getting traffic is up to you.