Cost of the card is usually $0-50 after a rebates if you sign a 2 year contract; if you don't sign a contract, then the card will most likely be $150-250; then you have to pay a monthly charge on top of that.
Usually any of the brands are good. I like Sierra Wireless
just pick the provider you want.
And yes you can tether with your iPhone 4 if you have AT&T and you pay the $20 extra fee. Personally I think that's a sham.