The general statement would be that it'd depend on the game, but that most will see more of a performance improvement from a better GPU than they will CPU.
It also depends on what scale you look at the CPUs in. The i7 2600K is faster, sure, but not exceptionally faster than the i5 2500K - only barely edging it out in plenty of tests. They're both effectively in the same tier/ballpark/whatever for performance. Realistically, there's not going to be anything where the 2600K will cut it and the 2500K won't.
If you're on a budget, and overall gaming performance is your priority, then putting the extra ~$100 into a better GPU is likely the better move.
As FFXIV already gets solid performance out of older i5/i7 CPUs with previous-generation GPUs, I doubt you have much to worry about. Get a single card to start with, however, as that tends to be the more reliable way for performance. Plus, you leave the option open for adding a second card down the line for an upgrade (assuming whatever you're running works well with SLI/Crossfire).
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