Simple. Unless you've got at least 2gb of ram (3gb recommended if it's not DDR2), don't put on Windows 7 or Vista. ALL versions of BOTH Vista and 7 have the "compatibility" mode that will allow you to run programs in a mock mode for Windows XP, 2000, 98/ME, NT, and Windows Server 2003:

Unless you've got some super old program from like 1992 (like Wolfenstein 3D or something), you won't have any problems with Vista or 7. I was actually able to make Wolfenstein 3D work in XP SP3, but I can't make it work in Vista or 7 for the life of me.
If you've got a Pentium 4 processor, it can handle Vista or 7, but I wouldn't recommend it. There are a few exceptions however. I have this awesome Micron computer with a 3.2ghz P4 HT processor and 8gb of DDR2 memory. That thing could handle Vista or 7 easily. But generally, unless you've got an AMD Athlon64 processor or better, I wouldn't recommend Vista or 7. With Intel, have at least an Pentium Dual-Core.
Also be aware that some specialized hardware might not work with Vista or 7. I had this one sound card made by Turtle Beach, and they went out of business right before Vista came out. The thing would NOT work with Vista even though it was only 2 years old!
If you want to upgrade your OS to Vista or 7, do NOT use the upgrade option. It's only a bit more work to do a complete format and it's well worth it. If you haven't backed up your programs and they'll be lost if you format, tough. That'll teach ya to back up your install EXE's. I don't even own a program I don't have backups to. If it's important enough to install and keep, then it's important enough to back up the install EXE to an external hard drive.
Drivers will be an issue after putting on Vista or 7. Just pray to God there'll be a driver for your network controller so you can connect to the net and get drivers after install. If there's not, you'll have to install a wireless USB adapter. Getting drivers can range from easy to extremely difficult. Normally if your computer's just an OEM such as Dell, Gateway, HP or other brand, it's easy as pie. Just do Windows updates and most of 'em install automatically. If you've got a custom build then you'll need a program like Driver Genius unless you want to spend forever looking up individual drivers for your components.
But there were NO driver programs for my HP Pavillion dv6000 Notebook on the HP site
Whoa man. If you've got a dv6000, get rid of it immediately. They have a known issue which causes the motherboard to fry. The replacement period is way up by now. Sell the sucker on Craigslist or something, 'cuz it's no use to you dead. Even if you had the motherboard replaced, it's gonna happen again 'cuz they used faulty motherboards to replace 'em!