My PNY 5800 Ultra fan has always performed like you describe--fan on for power up or 3D gaming, but never for 2D. It's not performing significantly differently than my 5900 Ultra engineering sample, so I haven't been tempted to buy a consumer version.
Out of curiosity, What happened to your 5800U? I had to RMA a PNY 5800U that Just died one day, with artifacts even on the post screen. Since then, I've put a fan on the passively cooled backside, and never had a further problem.
I used to drive a Heisenberg, but whenever I'd glance at the speedometer, I'd get lost.