i think you don't have any idea on what you're doing

you're problem is not with video drivers because they're already installed. let start by points:
1.-
;The comment below contains SFX script commands
Path=%SYSTEMDRIVE%\
Silent=1
Overwrite=1
as the first line says the comment contains
RAR SFX script commands. you must know how to create RAR-SFX, there are a LOT of posts talking about it and the piece of code i pasted is the
comment you need to add to the archive. i won't get far on this.
Quote
Also can I just place this anywhere in winnt.sif?
hell no!
2.-
Quote
DetachedProgram=cmd.exe
Arguments="/C START %systemdrive%\detached.cmd"
Also I guess I have to make a deatched.cmd and place it in the $OEM$\$1\detached.cmd correct?
that is ONLY if you're using the rar'ed compressed file, if not why add it?
3.-
Quote
Where is the file ATIRARSFX.exe?
lol, it's the SFX archive, if you didn't create it... how can you find it??
you must have a few knowledges until getting on this, i tried to explain it as easy as i could but you need to know a bit about how the process goes.
the final solution... look the estructure,
ATI\
-CP\
-Drivers\
place the ATI folder on $OEM$\$1\ and simply add this code to runonce
IF EXIST %SYSTEMROOT%\system32\atiddc.dll REG ADD %KEY%\055 /VE /D "Catalyst Control Panel" /f
IF EXIST %SYSTEMROOT%\system32\atiddc.dll REG ADD %KEY%\055 /V 1 /D "%SYSTEMDRIVE%\ATI\CP\Setup.exe /K" /f
that's all