Carbon Copy Cloner. Free, and kickass.
Just as an FYI, you don't actually need to install a fresh copy of Mac OSX on the 500GB drive....because CCC clones the operating system and basically everything on the partition that you are cloning from to the target partition.
I use it to clone my OSX installation all the time. As long as you disable "Enable ownership on this volume" on the target volume (Aka, the partition or disk you're cloning to), it will work just fine. Hell, it'll even tell you if the disk will be bootable.
Just a warning though, disable Indexing via Spotlight by going to System Preferences, Spotlight, and then adding the source volume (the partition or drive you're cloning from), and the target volume. Otherwise it'll keep trying to index the contents while it's cloning, and that will slow it down quite a bit. Otherwise it's pretty fast. Better than Time Machine if you ask me anyway, since it actually clones the drives contents rawly, instead of into an image file like Time Machine does.
Here's the scope of what you'll probably want to do.
1. Get an enclosure for the new drive, and place the new drive into it
2. Boot from the existing drive and download Carbon Copy Cloner and install it.
3. Format the new 500GB drive to HFS+ Extended (Journaled)
4. Right click the 500GB drives icon on your desktop, and click on more info.
5. Go down to the lock, click it, input your password, and uncheck the 'Enable Ownership on this Volume'. Not doing this step often results in permissions issues for some reason, which you do not want I can fully guarantee you.
6. Close the more info window.
7. Go back to System Preferences, and click on Spotlight. Click on the tab on the left
8. Add the 500GB drive and the current 80GB hard drive to the list, which will prevent Spotlight from slowing down the cloning process since it would be trying to index the contents of the drives while they cloned.
9. Start Carbon Copy Cloner from Applications
10. On the Source Disk drop down menu, choose the 80GB Drive.
11. On the Target Disk drop down menu, choose the 500GB Drive.
12. Make sure you choose 'Everything' on the 'What to Clone' dropdown menu.
13. Verify that CCC says that the Target volume will be bootable.
14. Click on the Clone Button
15. Put in your password
16. Wait for CCC to finish cloning the contents of the 80GB drive to the 500GB drive
17. ?????
18. Profit!