I would certainly upgrade to Win 7 over Vista or XP.
I played with the Win 7 Beta test, for which I cloned the main drives in both machines and "updated" to Win 7 Beta. I then used the various levels of the pre-release Win 7. There was no way to go from Beta to final release as an upgrade, because the Beta was the "top of the range with everything" version, and cheapskate Keef only bought Home Premium.
I didn't have any problems doing the final upgrade - I backed up, did a clean install, and then reinstalled the stuff I wanted. That cleared out all the old tat that had accumulated in both PCs. First time in about ten years and long overdue!
Win 7 was very impressive in the way it found drivers for all my peripherals - except a very old SCSI card used for the scanner and two CDRW drives. Win 7 doesn't really support SCSI. I ditched the CDRWs (new ones are much better, and cheaper), and bought a fine new scanner for not-a-lot.
The only serious problem I had with Win 7 was when I set up a "Home Group" on the desktop and laptop, and then realised I needed a "Workgroup". I deleted the Home Group on the desktop. That locked the C drives on BOTH PC and laptop! There was no useable way back from that, and I had to do the reinstall again on both machines.
Fortunately, a bootable Linux CD was able to unlock drives D and beyond. I can't remember now why it wouldn't do C.
I think (can't be sure) that Microsoft subsequently fixed whatever zaps all the C-drives on a network if the Home Group is turned off. I'm not going to experiment to find out.