Hello

I'm pretty new to retro PC hardware and was hoping to get some help. I recently got my hands on a HP Z557 Digital Entertainment PC. I bought it mainly to use as a media machine to play my 480p (or lower) media on my CRT TV or try and play GroovyArcade through it. I have other machines to potentially do all of that but they aren't really a "one stop shop" with their setups.

I don't think I Can upgrade everything within it, but if anyone has any experience with these machines or HP proprietary devices/cables, your assistance would be appreciated.

CPU: Pentium 4 530 --> I have a spare Pentium 4 651 but when I installed it, everything but the CPU fan would run.
GPU: HP Geforce 6600 --> I did some digging and it seems like the only upgrade would be a Geforce 7000 series card but I have no idea how to get my hands on it. The thing that sucks is that they have specific inputs that allow the GPU to access the Composite/S-Video/Component outputs. I also cleaned and replaced the thermal grease on the 6600 but I don't know if there's a fan replacement anyone can suggest.
PSU: It seems like the OEM one died on me. I heard the fan get slower, then the machine turned off. I've been trying to find a replacement for it, and of course, it's very hard to find as it has a specific orientation (PFC Connector). The model number is 5187-8274. I found a site that "sells" spare PC parts but they claimed it's on back order and they don't know when they'll get more, even though you're able to add it to a cart and pay for it. I am able to use an EVGA 650W but of course, it's missing one connector needed to power one of the boards in the machine (It's just a front composite input) but after it posts, it takes a long time for the MOBO splash screen to change. Once it does, it just goes to a black screen and I usually give up after 3 minutes.
MOBO: It's an ASUS PTGD-LR. I was able to get a replacement one just in case the one I currently have is causing issues.
RAM: Hoping to get four 1GB DDR-400 sticks. I figured that because I'm going to try and do more then just play media, I may need the full 4GB.
OS: I don't have the original recovery disk for Z557 but I found one online for Z558 and I'm hoping that is sufficient. I also downloaded an OEM Media Center 2005 Edition from Dell, MSDN and a regular Microsoft Windows Media Center 2005 Edition iso. I figure, I get the most out of each piece of hardware if I install the initial OS or something similar.

Not sure if there's anything else I should upgrade, keep or be on the look out for.