I decided on upgrade instead of clean install which took just under 2 hours on my super slow PC. That was trouble-free.
I ran the Windows Vista Upgrade Advisor first and printed out a copy of the task list which is a good guide of things to do after the installation.
Next would be a online upgrade which wasn't really complete because certain upgrades have failed. However latest device driver such as that for my VIA/S3G was upgraded.
Jobs' iTunes is a problem as neither I could start it nor reinstall it. Found that this is still an issue although you may try installing latest 7.1.1 but you could be prompted having vbscript issue which I couldn't yet fix. My current option is wait.
Another issue is printer driver from my HP LaserJet 1020, I'll have to use my notebook if I want to print at home. HP's website says driver may be available July 2007.