I'm trying to find an easy way to add all the post-SP2 fixes to ANY previously install Windows XP that has just had the XP SP2 update installed. Could I just use a simple batch file like this one:

start /wait Windows-KB890830-V1.2-ENU.exe /q /n /z
start /wait WindowsXP-KB867282-x86-ENU.exe /q /n /z
start /wait WindowsXP-KB873333-x86-ENU.exe /q /n /z
start /wait WindowsXP-KB873339-x86-ENU.exe /q /n /z
start /wait WindowsXP-KB885250-x86-ENU.exe /q /n /z
start /wait WindowsXP-KB885626-v2-x86-enu.exe /q /n /z
start /wait WindowsXP-KB885835-x86-ENU.exe /q /n /z
start /wait WindowsXP-KB885836-x86-ENU.exe /q /n /z
start /wait WindowsXP-KB886185-x86-enu.exe /q /n /z
start /wait WindowsXP-KB887742-x86-ENU.exe /q /n /z
start /wait WindowsXP-KB888113-x86-ENU.exe /q /n /z
start /wait WindowsXP-KB888302-x86-ENU.exe /q /n /z
start /wait WindowsXP-KB890047-X86-ENU.exe /q /n /z
start /wait WindowsXP-KB890175-x86-ENU.exe /q /n /z
start /wait WindowsXP-KB891781-x86-ENU.exe /q /n /z
start /wait gdidettool.exe /Q:A /R:N

Does the order of the hotfixes matter? Anything else I should know?

Running these hotfixes through a batch file will work fine except you need to download qchain.exe and run it after the last hotfix is installed.

This will allow you to install the updates with a single reboot and insure that the appropriate registry entries and system updates are registered.

I see this is detailed in How to install multiple Windows updates or hotfixes with only one reboot.

So, do I just make my last entry:

qchain.exe

Also, do I have the right switches? I get different options when running a hotfix with /? What does the /q /n /z do?

you can throw out this start /wait. if you add an exe-file to a batch-file it waits the execution time by default before starting the next entry.

the switches are correct. qchain.exe is no longer needed, too.

you can make it even simpler by using for-statements in the batchfile. example:

for %%x in (%SystemDrive%\Install\Hotfix\Typ1\KB*.exe) do %%x /q /n /z
for %%x in (%SystemDrive%\Install\Hotfix\Typ2\KB*.exe) do %%x /Q:A /R:N
for %%x in (%SystemDrive%\Install\Hotfix\Typ3\KB*.exe) do %%x

pay attention that i always rename the hotfix files from something like "WindowsXP-KB867282-x86-ENU.exe" to "KB867282.exe" and the for-statements above only work with files beginning with "KB" - as you may have seen already

I'm a little confused on qchain.exe. One poster says I need it. Another says I don't?

You don't need qchain. It's been unnecessary for pretty much all hotfixes released within the last year or so. Certainly for all post-SP2 hotfixes.