There was a time when my desktop, and those of my "clients", had a replacement motherboard about every couple of years. Occasionally the case/PSU had to be changed too. They gave significant improvements in performance.
O/S changes were treated more circumspectly as they were likely to be incompatible with existing peripherals or applications. The rule of thumb was to wait for version 3 or SP2.
The current PC is running W7 64bit on an i7-870 with 16GB. It's hard to see how to take advantage of the four processors - even the most cpu intensive application only uses one and a bit.
The upside is that there is lots of money in my savings account to buy something with a new processor/memory that would hopefully be much faster. I was waiting for W10 to settle down - but the nagware etc has put the kibosh on that route. I'm wasting too much time trying to keep my W7 clean.
The laptops for browsing etc will go to Linux Mint if W7 gets stuffed by the W10 nagware. The desktop big application will need a lot of work to port it to Linux - but it could be an interesting project.