The maximum enchantment bonus is +10 in the SRD, as in the game. See
here (third paragraph) for instance (the armor page says the same thing). The maximum is higher for epic weapons, I think.
Another significant difference from the SRD is that bonuses with the same named source don't overlap; the first bonus prevents all subsequent bonuses. So, by the SRD, if a character wears a +1 ring of protection (+1 deflection AC), and then casts a divine shield that would give them +4 deflection AC, they'd have both effects active, but only the best one would be applied, so they'd have +4 deflection AC over all.
In the game, the +1 ring prevents the +4 spell from taking effect, although spells can have partial results; for instance, a +2 strength belt blocks the +6 strength from divine power, but not the BAB or HP boost.