If you are new to Linux, you should not install Fedora Core 6 test 3, or any other test version of Linux. This is not a fully tested version of Linux. It is just what it says, a test release, meant for those with experience with Linux to test for Fedora and give feedback.
 
I am having a little trouble understanding what you did, please explain more fully.
 
It sounds like you did a text install and not a graphical installation? Did the installation complete fully?
 
According to ATI, it should be supported. See the ATI article here.
 
Could you provide a little more information?
 
Is this a non-athlon 64 processor in your system?
 
What is the make and model of your system?
 
Did you use the correct iso of Fedora for the installation? In other words, what ISO images did you burn to do the installation? The non-athlon 64 images, or the x_64 version for an Athlon 64 system?
 
Did you attempt to configure the video card and monitor during the installation at the hardware summary screen?

hi..i am using pentium4 3.2 with 1gb RAM. i installed it using virtual pc 2004. i didnt do any configurations to the video card and i think it should be full installation.i didnt burn the iso into dvd as i was using virtual pc as i mention b4. any idea?

According to this, FC6 T2 should work in VPC.
 
Can you make graphical installation?
Perhaps using following boot up command in the boot up menu (same place where you issued 'linux text'):
 
linux resolution=1024x768 nofb

i cant do graphical installation too.. that's y i installed text version and try. now i m downloading fedora4 and try.. thx!

Wilhelmus hit the nail on the head. I had a ton of problems installing Fedora Core 6 test 3 on my system, until I used the resolution command that he suggested to do the installation (not on Virtual PC).
 
Just be aware of the comments about the installation, noted in the link that he gave. It looks like, from my perspective, that Fedora Core 5 would fair the best.