Originally Posted by
Dandonk
Heighten affects the DC of the spells you cast, if they're lower than the max level spells you can cast.
If you're a new level 3 wizard casting your web, having heighten does nothing. Web is a level 2 spell, the highest you can cast already.
If you're a level 20 wizard, though, the DC of the web you cast with heighten on will be 7 higher than if you did not have it on - since your max level spells are no level 9.
The DC of your spells is: 10 + spell level + casting stat modifier + (items, feats, whatever).
Heighten will set the "spell level" part above to the max level of spells you can cast. Thus it will do little for your already high level spells, but will help a lot with web and other low level spells.
Heighten also costs extra sp per level heightened.