Here is a simple reason not to use the Quadro FX 5800:
It is of a much older design than the GTX 680. In fact, the FX 5800 is based on a GeForce GTX 285, with only 240 CUDA cores (and it uses only DDR3 VRAM). The GTX 680 technically has 1,536 CUDA cores, but is hamstrung somewhat by a 256-bit VRAM bus (albeit GDDR5).
so gtx 680 is not the best idea? does it fully supported my CS 5.5 / cs 6.0?
I have a question for you on the GTX 680, why do you and others say that the GTX680 is hamstrung by the 256-bit bus when the memory bandwidth of the GTX580 is 192.4 GB/s and the memory bandwidth of the GTX 680 is essentially the same at 192.2 GB/s. The specifications show that despite a narrower interface that have much faster memory for the 600 series (Data rate of 6 GHz versus 4 GHz) Incidently there is a serious memory clock rate mistake on the nVidia page on the GTX 680 specs. For backup I had to go to EVGA's specifications
anat_usa, go with the GTX 6xx family there is now a new GTX 670 with either 2 GB of VRAM or 4 GB of VRAM. You will be much happier in the long run.