Nothing wrong with a Wiz/Archmage, in fact that's a very solid combination. (for the purposes of things like DCs, spell penetration checks, your effective caster level can go beyond 40, as far as I am aware...unsure if spells that have no cap (i.e. Fire Bolt) scale past 40 in terms of damage dice/other variables, however)
For example, with Epic Spell Penetration, Focused Spell Penetration, Wizard35/Archmage5 (with 3 spell power feats assuming you take Master of Shapes and Elements), for beating SR, that's effectively 53 +d20.
Though I'm sure there are more potent combos (Red Wizard of Thay+Archmage, for instance) that can really break things. xD It's still fun to be able to laugh in the face of a Rakshasa as you blow past his SR.