Old workstations are power hungry so you want a 800 Watt to 1000 Watt PSU.
Get something that a minimum of 80 Bronze plus or better
The old XW8400 comes with two 64-bit Intel Xeon 5300 sequence quad core processors
But the 64 Bit version of XP have some driver incompatibility issues.
But you want the 64 Bit version if you want to use more than 4GB of Ram (actually more like 3.5GB as due to the limitation of 32 bit addressing.
Windows Vista can also run on the XW8400 quite well with less issues.
As well as various distros of Linux.
I would not go with any OS newer than Win 7 for Windows as Win 10 Home will not see both processors (Win 10 Pro would).
ATI Radeon X1950 XTX or a Nvidia GeForce 7950 GX2 were the top cards of the era (2006)
But I would not restrict yourself to period cards.
The CPU will be the bottleneck, so the more modern cards will not that well as you think with the PCI-E slot is only gen 1.
Gtx 660 or Radeon HD 7850 would be your best shot.
With alot of older games, they work better with the period graphic cards better as they get hardware acceleration, get something too modern and you stuck in software rendering
its up to you if you want to use the onboard Realtek High Definition Audio or something like a PCI or PCI-E Soundblaster card. A Expansion sound card can offload some tasks off the CPU and give you back a few frames.
I would use a SSD instead of a spinning HDD as SSD prices came down in price and should run fine in the old machine. It also help with start up and loading times.
As for Dos gaming you could do Win Xp, older machines running 98 will do a better job of this.