I use things like boots, belts and helmets to boost other things. I concentrate first on items which regenerate hit points and spell points. Also, I tend not to bother boosting any stats except speed too far above 50. The reason for that is that the next level above about 50 where increasing a stat makes a significant difference to your performance is IIRC 75, so that ofter a stat reaches 50, you have to boost stats a long way for comparatively little return.
I've just found the breakpoint table. A stat of 13 gives no bonus (stats under 13 give negatives!), and raising a stat to 50 gives a bonus of +9. To double that, you have to raise the stat to 275 – doubling the stat to 100 only gives a bonus of +11.
I'm not absolutely certain of this, but I think Speed influences who gets the first attack in, and in turn-based mode, I suspect that may be a direct comparison between your character's speed and your enemy's. If you kill things quickly, that really can affect the amount of damage you take. If you want to go further into this, you need to look at Bones' Combat Guide at the link below. I know the games pretty well, but Bones knows lots more than I do about how the games actually work.