I have just bought Quadro FX 4800 and trying to use it in Blender.
The 1st problem : I found that GPU render ( both support and experimental GPU rendering ) is slower than CPU render ( i use Athlon II x4 930 ) . It was a bit shock cuz i though it can never happen.
2nd problem : wenn i turn on GPU render, Blender crash everytime i switch into Live rendering view .
Can some one explain those problems for me ? 
i don’t want to waste my money on this Fx 4800
I hope you didn’t get the Quadro specifically for rendering.
A poor CUDA graphics card may well be slower than cpu rendering
http://www.systemagnostic.com/faqs/
Also, the 4800 only supports version 1.3 of the CUDA specification.
You should aim for a card that supports 2.0 or higher. Some of the features that Cycles uses aren’t fully backward compatible with 1.3 cards.
http://wiki.blender.org/index.php/Doc:2.6/Manual/Render/Cycles/GPU_Rendering
Depending on what you’re scene contains, lack of CUDA 2.0 support on your card could possibly be a source of some crashes.
thank you very much. i’m going to return it and pick a gtx card