In my opinion, a clean install is best. But as Arpan said, you will lose everything. I would suggest to backup all of your important data to another drive and do a clean install. But, again, the clean install will wipe out everything.
Clean install is always better. Will most likely get rid of any problems you are having with Windows. But, you'll lose all your data, so make sure you back up!
As everyone has said, upgrading will keep most if not all your settings and I think (someone call me out on this) your existing compatible programs. Preferably, a custom clean install is best. To me, that entails backing up vital user data, and THOROUGHLY reformatting the hard drive. Then install and reinstall everything needed. Best to get rid of gunk and start fresh!
The upside is with Windows 8, when you sign in with a Microsoft Account (Hotmail, MSN, Windows Live, Outlook) your Desktop personalization settings and some other settings as well as Start Screen colors and lock screen as well as pinned IE bookmarks on Start will roam with you from PC to PC. So by Windows 9 time or on another PC, you'll have most your settings with you on that PC. If you need certain Windows Store apps, your account will let you view your list of apps you've installed with that account in the Windows Store.
I'd say take a chance then go for it. Make a partition and install 8 on that and see for yourself what's best.
If you do an upgrade you will keep your hard drive files, though if you do a clean install it will make your windows 8 experience better. (quicker boot time, etc.)