first of all i'm sorry if in any way i have got off topiced or crossed the rules of the forum, but i would like to know if the  
256MB XFX PCI-Express GF 8400GS TV/DVI VGA Card is supported by the Mainboard Model P4M800CE-8237. Below I've taken a screenie of the specs of my computer for more detailed. Onces Again thanks to whom helps me out as im just a new guy in this world seeking info    
 
 
Specs for the 8400GS
Chipset :  8400GS  
SLi :    
Package Type :  Retail  
Chipset Features :  128-bit floating point High Dynamic-Range (HDR)
16x Anti-aliasing
Advanced Spatial Temporal De-interlacing
Microsoft DirectX® 10
nVidia ForceWare  
GPU Speed :  450 MHz  
Memory :  256/512Mb  
Memory Type :  GDDR2  
Memory Speed :  800 MHz  
Memory Bit Rate :  64 Bit  
Interface :  PCI-E (x16)  
Connectivity :  D-Sub
DVI
HDTV  
Cooling :  Fan  
 
Screenshot with my comp specs :-

The short answer:  No it's not supported.
The long answer:
You need an AGP graphics card for your motherboard. The graphics card you listed needs a PCI-Express motherboard to work.
 
On a side note: Is this a desktop computer or a notebook?

 
 
Thanks alot for the info and its a desktop computer

How big is the case?
Since it's a mobile-on-desktop platform my guess you also need a shorter bracket and half height PCB on your graphics card.

PCIe and PCI are not the same, and AGP and PCIe or PCI are not the same.  
 
Imo, you might want to build a new PC instead of even considering upgrading (possibly except RAM).