First I would do a test of RISing one machine at one time. Instead of doing a set. As you can image 500 machines at once I wouldn't recommand doing it in the real world.
My case senrio one machine at a time should always work.
As for the WINNT.SIF file be careful of this file. Make the WinNT.SIF file and save it after you made it to where you want it.
From my experience opening this file and/or moving / copying this file breaks the file.
Make it, save it, to where you want it and leave it alone from that point on.
I have created a CD called the AllState Common Enviornment CD 1.2 ACE 1.2 CD. What this CD is, is Windows XP Pro with SP2 slipstreamed with all the answer file / device drivers for our Dell's. This replaces the RIS server as we didn't have that great as luck as we wished.
Make a bootable Windows XP Pro SP2 CD and drop a WINNT.SIF file into the i386 folder. Once you done so you will have a Bootable Windows XP Pro SP2 CD just like you would RISing, but with a CD. Fail proof you just need a CD Rom. I will write more if you answer with questions.
All the machines that I have are able to boot from PXE. I have the RIS installed in my server and I have tried it, but somehow it changes the file extensions when it copies the i386 folder because the installation hangs at about 70% asking for files that are already in the folder and somehow the extension has been changed; like for exemple: (DLL to DL_ and stuff like that) when it searches for those files it can not find them, plus i have followed all the instruction placing the Winnt.INF file everywhere and I have not being able to make it work. I hate the RIS thing. Maybe I am doing something wrong, and by the way the CD use to extract the files from is an original XP_SP2.