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If its clocked to a decent speed, will it really be so terrible? As far as I can tell the main difference between a 2500k and an i5-750 is the bus speed. The rest should be reasonably marginal?
I'm so lost with this generation of hardware!
Well yes, at identical clocks they would be similar, but if it had been a 2500K you would ideally have it at 4.4Ghz or so. At the stock 3.4Ghz the 2500 is fairly long in the tooth now and you can see it in CPU heavy games (Playing Battlefield 1 64p conquest with the 2500, some maps the framerate hovers around 45-50).
Your i5-750 really needs to be closer to 4Ghz for it to be able to match even a fairly entry level graphics card in most new games.
In fact in a lot of games right now it is your i5-750 bottlenecking you, not the 280X which is still relatively decent (I was running BF1 at 2560x1080 high settings on one recently and it was a stable 60fps)
Getting a new card/adding a 2nd card would improve your visual settings obviously but you'd still be dealing with relatively low frames due to the i5-750.