Like CJM said, contacts, purchased apps, wifi connections are all associated with your gmail account. as long as you use the same gmail, you'll get those back.
the data within the apps is a different story. Some apps have a backup/export funtion built in so you can import your data on the new phone. A lot, like games don't. the only way you can backup and restore apps with all the data intact is by rooting your old old phone and using something like titanium to back it up. Other apps like My Backup Pro can backup some apps and will auto install them all for you, but only a rooted phone can backup and restore with all data.
You can certaionly use your old SD card, and I actually recommend it. If you upgrade to a new one, copy everything over to the new one. Keep your old one handy so you can reference what apps you have installed or make a list because sometimes free apps aren't saved in your "My apps" area of the Android Marketplace liek the purchased apps are.
