I would set the fan to high and then let the MOBO do the work. I believe the tri-cools simply have an internal resistor for speed control. Most fans without that simply run on max all the time and are controlled by the MOBO, so it should work.
If that doesn't work, you can always try a non speed regulated fan like the Slipstream as recommended above, which is a great fan.

HP Media Center machine
MOBO: ASUS OEM board
CPU: 2.6 GHz Socket AM2 Athlon X2 5000+
RAM: 2 GB Qimonda PC2-4200 DDR2 RAM
GFX: Sapphire Radeon HD 4850
HDD: Seagate Barracuda 320 GB SATA 7200 RPM