Having a touch screen is not really a major point since w8 desktop is exactly like w7. Only in metro, which is very limited in uses, will touch be beneficial. I have w8 on my laptop and w7 on my desktop. It really doesn't matter since you will be in desktop most if not all the time. However, you will need to get w7 pro to use more than 16gb ram. So w8 would be the cheaper solution.
It's not really a concern of compatibility but you need 64bit to address more than 4gb of ram. 32 bit will see all the ram and it can be used as a ram drive but it just can't use more as system ram. Cpu/mobo doesn't matter.