huh... in the picture there is a northbridge heatsink- i think you mean the southbridge
you should definetely put a heatsink on the southbridge because the plastic fan shroud on the 9800 gtx overhangs it and creates a heat pocket- some reviews online reported mobo crashes because of southbridge overheats even with a heatsink because there was absolutely no airflow.
Bare minimum, you should stick a heatsink on there, but make sure its flat because the card's shroud will be very close to it. If you have a little more cash, I would buy a small fan and do some kind of mod and attach it near it.
Antec's SpotCool fan allows you to position it anywhere thanks to the arm, but its not on newegg anymore, unfortunately.
And yeah, faster spinning fans wear out faster.

SONY VAIO PCV-RX755
CPU: Intel P4 2.0 GHz Socket 478
RAM: 2 GB Kingston ValueRAM DDR 333
DVD: Sony DRU-820A DVD Multi-Burner
Mobo: ASUS OEM board
GPU: eVGA e-GeForce 6200 AGP
HD: 60 GB IDE Samsung Spinpoint