I always run at least one OS behind - since the bugs are worked out. Thats why I am on XP right now. Will be upgrading to Win 7 in April. Win 8 is a pig - thats why they are trotting out Win 8.1. I won’t be upgrading to Win 8.1, because it is also bound to have bugs.
You need to first check to see that your PCs are Win 7 compatible:
http://windows.microsoft.com/en-us/windows/downloads/upgrade-advisor
I can tell you right now, you need minimum 1 Gb physical RAM - 2 Gb is better [about 50 bucks]. But, you also need to find out if the other components of your PCs are compatible. The upgrade advisor will tell you.
Here are some other specs for Win 7:
http://windows.microsoft.com/en-us/windows7/products/system-requirements
I played around with a refurbed machine with 7 on it for about an hour a couple weeks ago. I didn't have any major problems navigating it. As a matter of fact I could buy it for $125. But I'd have to put a modem in and it's only got a 80 GB HD. I'd also be very happy to pay $125 to MS if they would simply re-license XP and support it. That's not going to happen though. LOL
My system detail looks like this