GX400 here with 2GB of PC800 RDRAM, based on an Intel 850 chipset. in the CPU slot is an Upgradeware P478 with a SL7EY 2.8GHz Northwood chugging along... and an upgraded, beefier voltage regulator to handle the brunt.
There's a 305W supply from a GX280 shoved into the PSU dropdown, with adapters to get the right pinout for these early P4 systems. (this may change later.)
Other cards installed are a Turtle Beach Santa Cruz sound card (onboard is terrible hissy AC'97), an Adaptec 2940UW Pro for SCSI stuff, an Adaptec AHA-1205SA SATA card with a modified BIOS for better SSD and storage capacity support, and attached to that is a Crucial 275GB SSD and a 2TB Ultrastar just chilling out, because why not. Box is running XP.

Currently, the GPU in it is a Radeon X700 Pro 256MB.

The main usage case for this machine is going to be for drive imaging and SCSI device testing, but I do want to max the machine out to the absolute gills, no holds barred, within reason of the platform that makes it up.
as said, it has a socket 423 to 478 adapter already, and while the original bump from 1.8GHz to 1.9GHz felt like a speedup a year ago, going from 1.9GHz to 2.8GHz was like lightning. the X700 Pro seemed to perform better as well, but I'm still unsure if that's still the absolute best card to make use of the bottleneck I have which is the AGP4X slot.

The final thing that would probably give me the ceiling bump I want is going to possibly be hardware video decoding. I know this is *probably* possible in some way with the X1000 line, but only up to 720p. if I go HD2400 or higher, I'd get 1080p decoding support. so far as I know, Nvidia is off the table as the 6xxx/7xxx cards don't have any hardware video decoding support (I think?), and that came into play with the G9x cores in the 8xxx series, which isn't available for AGP at all.

I used to have an HD3650 AGP card way back 12 years ago, and it got killed during an unfortunate accident with a screw on a motherboard that was entirely my fault. if I still had this card today, I'd be in a lot better shape, I guess, to make the call myself. I also had an X850XT PE, but that didn't survive the move a couple years back and was found broken almost in half at the bottom of a box some months ago. The X700 Pro was another basement find and I don't remember where I got it from aside from having to do a full capacitor replacement on it with what I had on hand to get it going right. Later on, when money isn't too tight, I'm going to recap the entire system and see about modifying a better PSU to drop in for the sake of cleaner, not-20-year-old-supply power to what can amount to rather expensive parts in the here and now. Already verified that the voltage regulator for the CPU gets 12v power and not 5v, so I guess I'm doing a lot better than most Socket 423 owners, but the way the PSU sits in the case is rather weird and will require some outer casing modification to whatever goes in there later on. but I digress.

As far as what this would be used for game-wise, probably UT99/2004 and Quake 1/2/3 typical era shooters, and GTA:SA amongst some other titles from my youth that I never was able to really play. But, I do want the ability to decode 1080p HD video as both partly an ego stroker and--well, mostly ego stroking, and also to play stuff I have on hand of various blu-ray rips of movies I've done at 1080p... Perfect usage case for the 2TB drive aside from the vast multi-hundred-gig music collection and drive imaging. This might also get replaced with an SSD in the future as prices drop.

Is the X700 Pro the end of the line for this machine, or should I seek out a newer card (HD2xxx and above) that'd do what I want? is this even a go-big-or-go-home situation, despite the bottleneck of the slot? I know the HD2xxx series is hated by many due to various reasons, especially the 64-bit memory bus a lot of the lower end ones have (namely anything not an XT card on AGP for that generation) but would that even be a noticeable issue on an AGP4X bus?