suggest me that would i upgrade from 8400GS(512MB) to 8600GTS(512MB) or not. will it be better or not. i have xfx630i motherboard. i run viewsonic LCD VA1716w with resolution 1440x900 at 75Htz. i can afford 8600 but not more than it. or should i go to ATI series.
Well so it seems like you never really had a choice anyways and since it seems that you now 'have' to buy a 8600 then I don't think our recommendations would be rocket science.
But now that you bought it, never mind. If you do not play really stressful 3D games like Crysis or Bioshock, your GeForce 8600 GTS should do okay. Besides, you can always use a lower resolution/graphics quality and overclock.