Check my math please: Theoretically, if I have 2000 Intelligence therefore boosting my weapon DPS by 20 times, and 25% crit chance with 200% crit damage therefore giving me a relatively constant 50% DPS boost, then in order to achieve 20K DPS, I'd need a weapon with a rating of 666 DPS.
Your math is slightly off because main stat adds that % of weapon dmg onto the original 100% so you'd have a total of 2100%. Since your original question was how much weapon damage you'd need, it's 20000 / 1.5 / 21 = 634.9...
Confirm math: %Crit;%Crit Damage;Intel