I just added a Jaton PX8400GS-Ex (512MB RAM). It supports DVI, VGA and HDTV. Got it at newegg this week (about $50), so I'm still shaking it down.
Best upgrade I've done to the GX280: SSD for the boot/OS/pagefile. the box screams!
If you're still interested, I'll post updates after I add the HD Tuner.