Re: can anyone suggest me best hardware configration for combustion

No, the whole GUI, all the buttons and sliders and the rest of it, are drawn using OpenGL, all the time AFAIK. Its a recent revelation to me but it explains a lot of weird behaviour I have experienced on Mac over the years, with buttons and sliders not drawing properly on dialog boxes and such.
If you are doing a lot of 3D compositing, without RPF or any of that stuff, OpenGL will massively improve the level of interaction, too. I used to do a demo from a game FMV that had four or five layers and the whole thing ran real-time from the time I loaded up the footage until I started to use some of the RPF stuff [which doesn't show up with OpenGL, so I had to turn it off at that point]. I was moving, scaling and rotating layers whilst playing back at full frame-rate.
I wouldn't even try to use Combustion without a QuadroFX card and the 540 in my workstation was less than $300 18 months ago, so its not a huge expense. Its the compliant drivers that make the difference, more than the grunty hardware.

I'm a contractor for Autodesk at the moment but, unless otherwise stated, all opinions expressed here are my own.