What do you guys consider necessary unbuffed ABs ranges for frontline melee and support characters?
You'll be fine with base AB in the 40s, generally that works well enough, so don't get too crazy about it, BAB 30 +4 feats gets 34, say, a STR or DEX score of 30 is quite good, which adds +10, that would give an AB of +44, you'll normally need buffs, AA or WM to get your AB above 50. SO if you're not a buffing type, you have a high base AB at about +45
It depends if you are PvP or PvM.
Er... Then what do you think a good PvP or a good PvM AB is?
That again depends if you're allround, heavy PvP vs casters or specifically made for killing high AC characters (where you'd prolly go for cleric or AA or a combination). If you go allround, I'd go for Thax's advice, if you're going against casters or a generally non-high AC class, then of course you wouldn't need a high AB, but you could focus more on defense (DR/AC/Saving Throws/Immunities) or damage dealing.
Any values in the 40s are acceptable for melee, for regular PvM you might even stand with 30s, but anything below 35 is low unless you have buffs. Very high AC builds with buffs and all the stuff can get about 80 even in low magical environments, to hit that on acceptable range you'll need about above 68, and not being WM or AA means you're quite screwed (mostly) to hit often.
Don't worry that much, if you know you're going PvP where there are lots of high ACs then make a caster, it's a no-brainer, you'll never be able to beat everybody in PvP anyway.
If you are thinking only for melee PvP try to get over 45 I guess, say, self buffing melee non-AC-focused builds should get ACs in the 50s or 60s at best, not counting possible magical environment (which won't stack for most AC buffs, only Dodge AC).