Just for kicks I just ran the W7 Upgrade advisor.
It gave me the thumbs up for all of my software and hardware exept my Creative X-Fi sound card. It told me that it was incompatible with W7.
The funny part is that I'm running a clean install of Windows 7 Ultimate on this machine right now and that sound card works like a champ. At least I haven't had an issue yet that I know of.
One important part to keep in mind when chosing a Window7 version. The XP Compatibilty feature in Windows 7, from everything I've seen and read is only available on the Pro and Ultimate versions. What I'm reading into this feature, is that in order to insure full backwards compatibility with pre-Vista only software you would possibly need access to the XP Compatibility Mode.
http://blogs.zdnet.com/hardware/?p=4263
The only thing I can say with any confidence from my initial first hands-on with W7 Ultimate is that I've completed the installation of all of my old software onto this machine and have not run into any problems that I can tell so far, other than those noted below.
I
do not have the XP Mode installed. I made a deliberate decision this OS go around that if I had any software that would not install as is, it was going to get the boot and start new. Some of my software has been through a few too many system upgrades.
Two very notable oldies that went in without any problems were Office97 Excel and Quickbooks 5.0. QB5.0 was originally made for Windows 3.1 but was W95 compatible.
Also up and running with no known issues so far:
FSX
Steam
Empire Total War
War in the Pacific (Matrix)
Fraps
Cooltimer
The complete A2A FSX hanger
The complete Eaglesoft FSX hanger
The complete Carenado FSX hanger (minus the 172 skies & floats)
Coolsky MD-80
Dreamfleet Piper Dakota
CS C-130X
Lionheart's EpicLT
RealAir Duke & Taildraggers
FlyTampa - StMarteen
FSG complete
UTX Europe & US
ASA with XG
FSScreens
Paintshop Pro 9.0
Teamspeak
FSHost Client 1.1
Cooltimer
Ad-aware
Spybot S&D
CCleaner
Adobe Reader 8.0
Google Earth
Winzip 12.1
Odd notes:
1. I was going to install Office97 Word which is the only other MS Office piece I use besides Excel. When I took a look at the default Window7 Wordpad it appears to me that it is far more feature packed and capable than that ancient version of MS Word.
2. After holding off and reading about the new defrag capability and features built into the new Window 7 defrag utility, I decided not to install my commercial defragger.
3. One software utility that would not install is Norton's Internet Security 2009. Solved by Dl'ing the beta version NIS 2009-7 which is working fine.
4. The biggest speedbump had to do with the Asus software and utilities that came with my Striker Extreme Mobo. Not a single one of those items will install. Asus Probe, Asus AI Overclocker, Asus Updater and the chipset drivers. Most of this I would really have expected as they pertain directly to my very old mobo. Mostly foo-foo utilities anyway.

The chipset issue might have been a biggy but as best I can tell Windows 7 figured that one out on it's own.
FAC