Well the magic boost cap is 2900 and it doesn't matter if you have more than that, the game will count as if you have 2900, you can test it on dummies (try hitting it with a spell at 2900mb then equip something to go over it you will do same damage) but that's for targets that have 0 magic suppression.
Now is where you need more than the actual cap is to account for the magic suppression of the targets, for exemple if a target have 1000 magic supression you will need 3900 magic boost because it reduces it at a 1:1 ratio ( 3900 - 1000 = 2900 ). *same rules for PvE and PvP.
For some endgame bosses it's actually really hard to reach the cap normally or as you just starting out because they have a lot of magic suppression, so you just try to get as much as you can so you get close to the cap, also some classes have debuffs that reduce magic supression for some time so it helps.
*Magic Suppression of bosses examples : Hyperion = 1335 ; Grendal = 1150 ; Pashid ; 1335.
For Magic accuracy I don't remember the exact numbers but at 2k you are ok for everything for PvE (maybe a little more for ruhnadium/grendal), it depends on the Magic Resist of the targets so for PvP vs a class wearing an MR set you will need more. (there's also MA bonuses or penalties on some spells that also goes into account but that's going too deep).