Increasing your DEX won't really
aid the ability for your ACC gear to take effect. Let me start from the top--there are seven base stats:
STR
DEX
VIT
AGI
INT
MND
CHR
These stats affect things in a linear way, like so:
For example, the difference between 50 STR and 70 STR is the same as the difference between 70 STR and 90 STR, so you can add STR to your heart's content and always see results. Now beyond these stats, it gets a little mirky:
STR affects ATK & RNG ATK
DEX affects ACC & Critical Hit %
VIT affects DEF
AGI affects Shield, Parry, EVA (Guard too?), RNG ACC, & Critical Hit % against you
INT affects Elemental Magic, ability to enfeeble with certain spells, & resistance against Black Magic
MND affects Cure Potency, ability to enfeeble with certain spells, & resistance to offensive White Magic
CHR affects Charm Duration & Success Rate of Song Enfeebles
I may have made a minor mistake or two in there, but bear with me. I also excluded a few things like "enhances some weapon skills" because that's not really a stat. Alright, so the problem with this new set of stats is their graphs are not linear--they are exponential, like so:
So the first few points you add to it are great, but as you add more and more you realize that it's not helping as much. For example, the difference between 200 and 300 ATK is probably about the same as the difference between 300 and 500 ATK. You need to add more and more to see fewer and fewer results. Eventually you come to a number where it's just not helping to add to the numbers anymore (unless you're facing a level 90 HNM, in which case you probably will never get anywhere near the relative maximum of the curve).
I know more about STR and VIT so I will talk about those for a moment. Let's say that you are in an XP party hitting Robber Crabs for 70-100 per hit (not counting criticals) and quite frequently hitting the crab for somewhere near 70 and somewhere near 100. That's quite a wide range so you would probably benefit from more ATK. This will raise your average damage. The potential to hit the mob for 70 is still there, but rather than your hits averaging to be 85 per hit (right in the middle of 75 and 100), it might be something like 92. You will constantly hit in the high 80s and low 90s now, with the rare 70-something popping in. Now, let's try a different scenario. Let's say you are fighting Catoblepas and you are hitting them for 110-120 per hit. You probably have an overabundance of ATK and would benefit from adding STR. Let's say you remove 20 ATK and add 5 STR. Firstly, the -20 ATK will make your average something like 112 instead of the 115 it was previously. Then the 5 STR will raise the entire curve itself, to something like 115-130. You have just changed your curve from 110-120 to 115-130. There is such thing as too much ATK.
If you want the VIT/DEF version of what I just said, substitute VIT for STR, DEF for ATK, and Robber Crab and Catoblepas for "you".
That's basically all I can say about this.^^