You might want to give some thought to anything you want to use the computer for other than just Sonar X2.
Since you're thinking about upgrading an "older" machine, look at your existing hardware and make sure you can get OS-specific drivers for any hardware you plan to keep.
Same goes for "older" apps - if you're seriously planning on upgrading to Windows 8, make sure any software package you plan to keep will run correctly under Windows 8 (Sonar X2 aside, a program running correctly on Windows 7 is no guarantee it will run correctly on Windows 8 ~ check the Microsoft compatibility web site to be sure).
Purchasing Windows 8 will be cheaper than Windows 7, but taking care of the rest could turn out to be MORE expensive if you have hardware and software compatibility issues and have to unexpectedly replace more than just the OS.
FWIW, a lot of folks here (myself included) are running X2A on "older" machines with Windows 7 Pro 64-bit and 8+ GB RAM, and aren't having any issues. You could always start out that route, and add a dual-boot partition to run Windows 8 at a later date (that's my plan...)
Randy