My system:
MoBo: ASUS A8V-MX
CPU: Athlon 64 3000
RAM: Kingston 1Gb
graphic card: Nvidia geforce 6200 AGP 256Mb
OS: Kubuntu 6.10 (kernel 2.6.17-10-generic)

Well,I've installed the latest proprietary driver, began to play a little with Flightgear and checked with nvclock_gtk the GPU temperature. It was 95°C!! Its temperature under no load is around 65C, which I think to be high. I don't think that it's a problem of crowded case, because CPU and MoBo temperatures are now at 35C. I've removed the case cover and the problem still continues. And I don't think that it is a problem of this card, because this is a replacement for another that has shown overheating symptoms. Further, it is very slow. fgl_glxgears shows 10FPS!!

/sbin/lspci|grep VGA

01:00.0 VGA compatible controller: nVidia Corporation GeForce 6200 (rev a1)

anything interesting in Xorg.0.log and glxinfo
DRI is working

I wouldn't do that. Good case covers are built so they take in air from place X and put it out at place Y, circulating the air so that components get cool air and pass hot air. It means the case is ventilated; if the cover is removed, air flows in and out where it wants to, and not necessarily cools as well as with the cover on. This, of course, depends on how you've set up your fans and other cooling devices..

If your glxgears is showing 10fps (come on, my software driver gives 170 on a bad day), there's definitely something wrong in the driver-X-software-line. I'm not surprised if your card turns hot if it's working so hard. Before gaming start figuring out why you're not getting proper fps. What does it look like if you use the non-3d driver for nvidia cards that comes along X (was it 'nv' or 'nvidia'..something anyway)? Have you enabled glx and other stuff in your X configuration, and so on?

Thank you, b0uncer. Removing the case cover was only a kind of emergency procedure, because 95C (65C when idle) is too hot. The design of the cover is optimized only for the CPU fan and the CPU isn't getting much hotter (keeps at ~30C).
Using nvclock I've found that AGP was disabled, so I've changed the option "NvAGP" in the nvidia driver configuration from "1" (use nvidia's AGP driver) to "3" (try first to use the driver in the kernel). Things are a little better. FPS in Flightgear reached two digits and glxgears now runs 7x faster, making 70 FPS (~90FPS when the system is idle)... Of course, things are still wrong. Still with the case opened and without external fan, the temperature is 57C when idle and 79C when gaming.

These are the relevant parts of my xorg.conf:

Section "Module"
Load "i2c"
Load "bitmap"
Load "ddc"
Load "extmod"
Load "freetype"
Load "glx"
Load "int10"
Load "type1"
Load "vbe"
EndSection

(...)

Section "Monitor"
Identifier "L1753T"
Option "DPMS"
EndSection

Section "Device"
Identifier "NVIDIA Corporation NV34 [GeForce FX 5500]"
Driver "nvidia"
EndSection

Section "Screen"
Identifier "Default Screen"
Device "NVIDIA Corporation NV34 [GeForce FX 5500]"
Monitor "L1753T"
DefaultDepth 24
Option "NvAgp" "3"
Option "ConnectedMonitor" "DFP"
SubSection "Display"
Depth 1
Modes "1280x1024" "1024x768" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 4
Modes "1280x1024" "1024x768" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 8
Modes "1280x1024" "1024x768" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 15
Modes "1280x1024" "1024x768" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 16
Modes "1280x1024" "1024x768" "800x600" "720x400" "640x480"
EndSubSection
SubSection "Display"
Depth 24
Modes "1280x1024" "1024x768" "800x600" "720x400" "640x480"
EndSubSection
EndSection


The card name is obviously wrong, but AFAIK this doesn't matter.

Thank you again

It does not really help, but it is normal that graphic cards get hotter than CPU's these days.