I am using an old PC Pentium4 2.4Ghz with nVidia GS 7900 AGP 256MB and I would like to support the video play from GPU. As far what I have read, the support for vdpau has come for nVidida 8 series. Is there any way to support playing from GPU using mplayer? 
How to get GPU acceleration working for my old PC?
linux;mplayer;gpu
Your video card does not support VDPAU but it does support XvMC, which is a precursor to VDPAU.  XvMC is notoriously difficult to get working, it is not as powerful as VDPAU, and it only supports acceleration of MPEG2 videos.  However, it is the only option for accelerated video playback for your GPU and Linux. The MPEG2 format is a an older format for compressing video files.  It is not as efficient as more recent formats, such as MPEG4.  Therefore, videos that have been encoded for distribution through the internet as well as many handheld video cameras and cell phones will typically be MPEG4 files.  That being said, MPEG2 is widely used for broadcast television, both over-the-air and digital cable, as well as DVDs.  Therefore, if your source of video is from TV or DVDs, then it is possible to play it back using XvMC.The way to enable XvMC depends on your Linux distribution and requires the nvidia proprietary video drivers.  Resources for installing these drivers and enabling XvMC for popular distributions can be found on various wiki:Arch LinuxFedora and Ubuntu (via the MythTV wiki)GentooOnce you have the nvidia driver and enable XvMC, you can use it with mplayer withmplayer -vo xvmc <video file>