Looking for CR balance...

[Kerilstrasz]

A human Wizzard npc of level 6 without templates or other +LA or +CR mods, is considered to be a CR 6 encounter(by class lvls)or CR3(human 1/2 CR * 6lvls =CR3)??

Regardless the above answer...
A human Wizzard npc of level X without templates or other +LA or +CR mods,
is a "specialized summoner" that in a battle he goes Impr.Invi. and summons his powerfull monsters as a usual routine... does that increase his CR(him and summoned monsters)? increase maybe the battle CR? or nothing of the above?

[Telonius]

He's considered a CR6. I don't know if it's explicitly spelled out anywhere, but most of the PC races are compatible with any of the PC classes, so any regular levels gained count as associated class levels (http://www.d20srd.org/srd/improvingMonsters.htm#addingClassLevels). "Nonassociated class levels" (the kind that count for 1/2 CR) are really only for giving enemies NPC levels (Adept, Aristocrat, Commoner, Expert, Warrior) or giving them a class that doesn't help them much. Like giving an Ettin a level in Bard or Wizard, or something like that.

Summoning an extra monster doesn't actually count as a higher CR. CR is - at least theoretically - an estimate of the enemy's total threat to the party, based on all of the resources the enemy has available. The DM can always adjust the experience reward, if the enemy's abilities are perfectly suited to the surroundings. (Fighting Tucker's Kobolds in their natural environment, versus fighting them randomly in a forest, for example).

[Roguenewb]

If you want him to be good at summoning against players you need the alternative class feature that lets you summon as standard action:

http://www.d20srd.org/srd/variant/classes/specialistWizardVariants.htm#rapidSummoning