Google Voice would be ideal but as an alternative you can port the number over to Ooma. I've been using the service for about a year and have been relatively happy.
Depends if you can afford the upgrade. Want an iPhone 3G S? Already have an iPhone 3G? Be prepared to pay $700 to upgrade [Updated] An iPhone 3G customer in most cases can early upgrade at $399 [16GB] or $499 [32GB].