You can't use an i7 990X (1366 Socket CPU) processor on the Rampage IV boards.
Their not the same socket.
The Rampage IV boards is a LGA 2011 socket and IS NOT a LGA 1366 Socket
There are only currently 2 CPUs can can be used with the Rampage IV boards these are:
- i7 3930K ($600) 6 Cores 12 Threads 12MB Cache
- i7 3960X ($1000) 6 Cores 12 Threads 15MB Cache
There will be a cheaper 4 cores 8 Threads CPU due sometime Q1 this year, it should be around the $300 price point, I believe it'll be called the 3920K.
Now if you meant to replace the motherboard with a Rampage III Extreme or the Rampage III Black Edition then sure you can use the 990X, if that was what you meant to ask.
I'm 100% in what your asking. Your saying you want to replace your P6T setup and looking at a Rampage 4, but you want to look at the 990X which isn't even the same socket as the Rampage 4. Which ever is the case the $600 i7 3930K out preforms the older i7 990X which is still a $1000 processor.