I've been pondering the same type of questions, and was going to crunch these numbers eventually(week or two probably) - so why not now.
There really aren't too many things that hit hard or often and do poison damage or cold damage. Harm, and wither do cold damage, maybe a mystic or weaving spell too, but that won't come up in PvM. For poison damage theres... the poison spell, poison strike, and strangle(I think). Since you're a sampire, you'll only even be affected by the bigger poisons, but with 70 resist, they do about 13 damage / tick, so with 68 that might go up to 14...
Lets look at some real numbers. Per Stratics, either poison strike or wither top out at about 55 damage.
If you have 70 resist you'll take 55 * .3 = 16.5 damage, so probably 17.
68 resist: 55 * .32 = 17.5 ~18.
Any attack that does <50 damage before resists will do either the same damage or 1 more, depending on rounding. Of cource this holds true for something that hits with all or part poison damage too - you're likely to take an extra 1 damage per hit, maybe 2.
Now looking at DCI.
Lets assume you're fighting something that hits hard enough to really make a difference. I'll assume you're collecting barbed hides, and you come across a g-drag and want to kill it.
Base damage: 24-33, with an average of 125 tactics, and ~1200 of STR - giving it a damage bonus of about 700%. (100% physical) So we will say it's damage range is 190 - 250 with an average of 220.
220 * .3 = 67 damage (I find myself in thie scenario quite often, so and this jives with what I notice, I'm always on swamp dragon with armor and GDrag hits me for 50ish per hit)
GDrag has avg 130 wrestling skill (per hunters guide). I assume he has 0 HCI.
So based on the hit chance % equation, if you have 0 DCI he has a 53.57% chance to hit - we'll round to 54. Then for each increase of 1 DCI he loses about .5% chance to hit.
Lets look at what this means for DPS (or in our case per 1.25s combat round). Where DPCycle = Avg Damge * Chance to hit.
At 0 DCI you can expect to take an average of 67 * .54 = 36 damage.
Take this to 45 and you're down to 25 damage.
At 50 DCI its 24 damage.
At 55 DCI its 23 damage.
At 70 DCI its 21 damage
BUT
at 70 DCI that means you only have 65 physical resist... so instead of each hit doing an average of 67 damage, it does 77 ... so recalculate
at 70 DCI its 24 damage. - Interesting, 70 DCI and 65 resist is effectively the same as 45 dci and 70 resist. I suspect this is not accidental.
Lets look at Bealank's suggestion: 75 resist and 30 dci.
220 * .25 = 55 (hmm - already looking better).
With 75 resist and 0 DCI you can expect 30 damage.
At 20 DCI (30 cap, but under -10 from Divine Fury): 25
At 30 DCI: 23
No need to go higher, since this is our cap.
Oh, and since you've moved up to all 75s in resist, that wither/poison strike from above now does 14 damage instead of 17.
So, what does this tell us?
Looks to me like the conclusion is that you're better to have the resists. In true physical combat it basically washes out. Max DCI and Min resists is merely a rounding difference between Min DCI and Max resist - so on any given hit, the difference is only 1 damage. However, because a lot of things cast spells...
You're better off to have the resist.