The part number I need is HP 5187-8274.
I'll say what a ton of people have already said: I love this thread. I need to thank everyone for putting their insight together. The idea of rehabilitating old hardware is always appealing to me, and when I found a beat-up Z552 at the Salvation Army for $20 (they thought it was some kind of DVD player) I knew it had to be good for something. I figured out the power supply was blown, but I had a big 525-watt psu sitting around. I've got it hanging out of the media center's back... pretty ugly but it's not like it's visible day-to-day. Put in 3 gb of RAM, installed Windows 7, and it's really running pretty smooth by now. I just installed a new graphics card, though, and I'm having some odd problems.
The card is the NVIDIA Geforce GT 430, which I've seen a couple people recommend. Video output to my HDMI-equipped CRT television is looking great, but I can't get the card to push audio through the HDMI connection. I'd just use the onboard audio jacks, but there's that weird thing where audio's routed through the original graphics card via an 8-pin cable that isn't compatible with the new graphics card. So that's out.
I've installed the latest drivers for the video card. When I go into the Sound control panel I don't see an option to select HDMI audio output - just the "Speakers" output that previously meant audio out through the RCA jacks, and an SP/DIF option for an optical-enabled system I don't have. I can't see anything about It seems like the drivers didn't install when I ran the installer program I downloaded from NVIDIA's website... I can't figure out how to install them manually, though.
If anybody's reading this who's gotten audio to work with the GT 430, I'd really appreciate some pointers.