While there is a lot on the internet about how voodoo 2’s run hot, different methods of cooling them and how P4’s and above cook them nowhere actually gave specific temperatures!

Over the past 3 months when home I have taken a few measurements. It is by no means a conclusive test. But gives an idea average temperature to expect (in Celsius)
Hardware has remained the same but the PC has moved location and the top Voodoo had to be re seated so fan has moved a little, nothing is overclocked.
All tests were run at least 1 hour, often longer. Results were rounded off to the average full degree

Hardware used
CPU: P3 Coppermine 1Ghz 100 FSB
M/B: Intel SE440BX3
Video: Albatron Gforce Ti4600
Voodoo2: x2 Diamond Monster 12MB
Fan Controller: Scythe Kaze Master 2
Fan: Akasa 20.92 cfm 70mm 3200 RPM fan (Taken from a unused Socket A heat sink I had)

T1= Ti4600 (as a reference card)
T2= Top Voodoo, Sensor in centre of FBI chip
T3= Bottom Voodoo, Sensor in centre of FBI chip
T4= Ambient, Sensor is loose in the bottom front of the case.

Results
T1 T2 T3 T4 Status
43 48 50 21 Idle
49 72 73 22 NFS3
46 70 70 20 NFS3
46 51 52 23 ROTT
51 79 79 24 Unreal TimeDemo
52 77 75 23 Unreal TimeDemo
42 63 57 26 Unreal TimeDemo continued from above with fan now on
39 43 40 24 Idle with fan on
41 63 56 25 Unreal TimeDemo with fan on
39 45 40 20 Idle
44 50 47 20 Idle continued from above with fan now on
46 51 47 21 Idle
43 47 47 21 Idle continued from above with fan now on
49 72 72 22 Unreal TimeDemo from above with fan now on

Conclusion
It seems when in use the card will generally run about 50 degrees hotter than ambient.
My fan was between the cards and the case side so not much airflow this still reduced temperatures by 10 degrees on the top card and a further 10 degrees on the bottom card. This would definitely improve with better fan position.
The last set of result was strange as even with the fan on the temperature was matching temperatures set without a fan. By this time the PC had been on and idle all day. Every other test was done in the space of a few hrs then shut down.
Maybe it’s just one of those things. I did notice one time while playing Warcraft 2 with the fan off the cards were running around 70 degrees, too hot as they shouldn’t be doing anything! Turning off and on fixed that and hasn’t happened since?

Picture of the test system. The large fan and 2 sitting on the case floor were not used during testing.

I dont intend to do any further testing for awhile. I do intend to keep temperature sensors on my voodoo's though and would like to see what they do in my P4 sometime in the future.