I have a desktop loaded with memory and a fast processor, running Windows 8 64-bit. It ROCKS.
You MUST do your homework on drivers for whatever audio interface you plan to use with whatever computer you plan to pick up, as some interfaces definitely do NOT have drivers for Windows 8, and some do not even have them for Windows 7. You must also check that the drivers cover 64-bit too.
If your interface is using USB 2.0, then you will most likely want to make sure you have one or more USB 2.0 ports on the new computer, as some interfaces freak out when they are plugged into a USB 3.0 port.
Other than the above considerations, both Windows 7 and Windows 8 are FABULOUS for running Sonar. Windows 8 has been quite stable for me, and Win 7 was as well. There IS a bit of a learning curve if you move to Windows 8, but it is really no big deal, although some folks meltdown dealing with the Metro UI. I actually like Windows 8 a lot now.
Good luck - get lots of memory and a fast processor and pick up at least 2 drives if you can, to split the OS and programs from projects and samples, and if a 3rd drive is available split the projects from the samples as well. You might want to also pick up a USB 3.0 external drive for backups too.
Bob Bone