That figure will depend a lot on what you are doing. For example, irules need CPU cycles to execute. Each connection is kept in the connection table needs memory.. the amount of memory you have will matter here. TCP buffers, will use memory... persistence records... so it is hard to arrive at a hard figure here.
The 3900 and the 2000s look pretty identical in terms of hardware, barring any architectural improvements that might mean slightly higher performance from the newer 2000s, so you might expect the same or better from the 2000s