Make sure there's 64 bit drivers for all your hardware (in case you're using something that discontinued) and that they also have support for the newer Windows version you're planning on using.
Once you install your new Windows (7, 8, or 10 etc...), you will unfortunately have to install all your software (VST/plugins) separately.....(so, you won't be able to "transfer" any of your software).
You'll have to check all your 3rd party plugin vendors to be sure they have the 64 bit plugs available. If the 64 bit plugins aren't available, Sonar will automatically use a program called Bit Bridge. This way, you can use any 32 bit plugin within the 64 bit environment. There's also a paid version called
JBridge (about $16) which is more reliable than Bit Bridge. Generally though, most plugins should have their 64 bit versions.
You will also have the two ‘Program File’ folders that are listed in your C: drive (OS drive).....one for 32 bit and one for 64 bit. The folder with 'Program File (x86)' is the 32 bit version......so if you want to use 32 bit plugins, they will go here.
I personally try to keep everything on my machine only 64 bit, but sometimes, there are some really good 32 bit plugs that need to be installed.
It's exciting stuff to get a 'new' machine (basically, like starting a new) built......the good thing is, now you can weed out all the useless stuff you barely used and install only the software that you really use regularly (until GAS gets the best of you and this vicious cycle begins all over again).
Best of luck RD!!