You came on asking for advice on how to get 120fps with your budget. People suggested how you can reduce some parts of your rig to improve other areas to get you closer to your goal.
Think of it this way:
In an imaginary computer, you have 5 components. In this imaginary world these components are all equally useful for frame rates, and all scale nicely with price (so a £100 component is twice as good as a £50 component, a £200 component is twice as good as a £100 one etc)
In an ideal world, therefore, if you had a £500 budget, you'd have 5 components at £100 each to give the best possible computer for your money. If you had twice as much money, you'd spend twice as much.
What you're suggesting instead is a setup where:
3 cost £100 each
One costs £150
The last costs £50
This means that your computer will run at the same speed as a £250 computer, even though you've spent £500 on it, because the other 5 components are waiting for the £50 one, so you'd fix it by getting rid of the expensive and cheap components and buying ones that fit your system better.
In the real world, things aren't as nicely balanced, but the same theory applies that a rig with some over-specced components mean that you're going to have expensive kit waiting around for something to do, whereas a rig with some under-specced components mean the whole rest of the system is going to spend a lot of time idle. What the posters in this thread are trying to do is suggest how you can balance the rig to achieve as closely as possible what you want to achieve, with the budget you have, by balancing the rig better than your original suggestion.
As for expansion and upgrades, it's not the right way to build a computer to unbalance your rig now in order to upgrade it later - by the time you go to upgrade your CPU, your memory will be out of date and you'll always have one part or other waiting for the rest. Buy a good quality case, power supply and hard disk drives (as these things need to be upgraded far lest often). Then spend the rest of your current budget on a balanced motherboard/memory/cpu/gpu. When it comes to upgrade time, sell all 4 of those items and put the money toward your upgrade. You'll have to top it up, the same as you would be with a single component, but you'll have a better computer at any one stage.