you have enough room concerning height above the mainboard (= space between mainboard and case side panel) you could use a 120mm fan and use an 120 to 80 mm adapter to install it to the heatsink. The advantage of the 120mm is that you can run it with less rpm = less noise because the 120 mm can move the same amount of air with less rotation speed.
You could use Scythe Kama PWM
as the fan (it will be regulated by the mainboard) and an adapter of this kind:
Thermaltake UV Blue 80 to 120 Fan adapter with Screws A2380
The above proposal asumes that Compaq didn't modify the BIOS of the Asus board so the PWM fan regulation was adapted exaclty to the fan delivered with the system. If this is the case you'll have to drop PWM regulation and work with fixed rpm.
You could also use a 80 mm fan but I am very sure that'll never get as quiet as it could with a 120mm fan. For your 80mm fan you'll need much more rpm to cool down your rather hot Intel P4 processor.
Gaining real advantages in noise level while staying with 80 mm will be problematic with your P4 I'm afraid.
In 80 mm I found these:
Arctic Cooling AF8025 PWM, 80x80x25mm, 600-2000rpm, 46m³/h, 10-21dB(A)
Arctic Cooling Arctic Fan 8 PWM, 80x80x38mm, 600-2000rpm, 48m³/h, 9-20dB(A)
Enermax Twister Cluster 80x80x25mm, 500-2000rpm, 19.23-55.05m³/h, 8-19dB(A) (UCCL8)
I know, i know - everything is rather complicated...