To find the absolute value of Jordan's new account balance, we start by calculating his account balance after the purchase of the computer.
Jordan had $400 in the bank. After buying a computer for $450, we subtract the cost from his balance:
\[ 400 - 450 = -50 \]
This means his account balance is -$50, which indicates he is in debt.
The absolute value of his account balance is:
\[ |-50| = 50 \]
Thus, the absolute value of his new account balance is \(\boxed{50}\).