Just had a look at my Dell Dim. 8100 (P4 1700)and the air flow is very low, 15 - 20 cfm I would guess. For a Dell P4 machine 40 cfm with dedicated CPU air ducting would be way too high and noisy. If you buy something in the mid-twenties you should be OK. (Monitor your CPU temp though).
If you have a third wire its usually for data aquisition; nice but not needed.
Your case ventillation is not solely carried out by your PSU cooling fan, the case ventillation load is shared with the PSU fans.
Trust this helps.