This will be a short post as I realized I lost my notes on the steps I took to make this work.  But I do have the modified BIOS I created and a link to at least one of the helpful pages that are out there.  You'll have to Google for the BIOS modification tools yourself.

I originally liked the old i945GTt-VFA motherboard because it had an Intel Pro/1000 which was comforting because before relegating it to being a Media Center PC it was used to run OpenSolaris and there was no question of LAN driver support.

So what's Etherboot/gPXE?  It's a network bootloader that supports pretty much everything you might ever want.  Check it out here.

Back to my little Media PC.  My eventual goal is to run the MCE PC without a drive by booting directly from the BIOS off of an iSCSI target and then loading Media Center (Win 7 Ultimate). 

Having a BIOS with iSCSI boot built-in gets me part way there and that's why gPXE is interesting.

I've taken a break from pursuing that goal for now because I'm not longer using OpenSolaris and its awesome iSCSI + ZFS support on my development server.

But, I did get as far as adding a gPXE BIOS addition to the AOpen's BIOS and replaced the default PXE boot code.

As I said, I no longer have notes on how I did this but this should get you started: Adding gPXE to BIOSes

Also, be aware that you do not necessarily need to flash your BIOS to enjoy gPXE.  You can just as easily boot a gPXE bootloader from a USB key or load it through your existing BIOS's PXE loader (aka chainloading).

Some rough notes:

1. I built my new BIOS in mid-August 2009.
2. I had issues with Rom-O-Matic generating a ROM "stripped" of everything but what I wanted and below 64KB.  There are definite size limitations in some BIOSes.  You may have to get your gPXE image below 64KB.  Perhaps that's fixed now.
3. I wound up installing Ubuntu in a VirtualBox and building gPXE with the correct flags to strip as much as I could and was able to get it down to 62KB.

For those of you with an i945GTt-VFA, I've attached the BIOS that I built on Aug. 16, 2009.  It was based on the version 1.07 which was the last BIOS released by AOpen for this board.  Once flashed, if you enable LAN Boot in your BIOS settings you'll see the gPXE addition in all its glory.

Disclaimer:  Don't flash this unless you know what you're doing!  You might be better off starting from scratch like I did so you understand what's going on.  Or, even easier, try booting gPXE from a USB dongle before taking the next step.

i945GTt-VFA v1.07 + gPXE iSCSI