I think I'd still upgrade the CPU first because the 2600 is not especially capable by comparison and the fact that you have a 100hz monitor says that you're going to want that capability in order to try to sustain reasonably close to 100fps. Honestly, with that ultrawide resolution of 1440p, you're pushing about 25% more pixels than a standard 1440p display and it may be necessary for you to look at upgrading both at some point depending on your requirements or whether you're ok with some lowering of settings. If you do lower settings, then the CPU is going to become an even bigger consideration.