If it is a Pentium 3 server using RDRAM then it is surely one ugly beast (Intel i820 wasn't that great as a chipset). Assuming it is even if you upgrade the video card (I suspect it has an AGP 4X Pro slot, which sould take that video card) it will not perform well in todays games at all. (Even a Radeon 9800 Pro, which is twice as powerful as the 9600 Pro isn't that great these days).
Have you tried openning the case and looking for an AGP 4X / AGP 4X Pro slot ?
ED: Well, with a Radeon 9600 XT, assuming it has a AGP 4x slot, it'll run Quake 3, Return to Castle Wolfenstein, etc (including games over 2+ years old) pretty damn fast.
It would struggle with Half-Life 2 / Source if it is a Pentium III, and I wouldn't dare try Battlefield 2 on it.
The board has a 4X slot, if the ATI supports 4X it should work. Not quite sure why you would want to run this card with a Pentium III Slot 1 CPU though, seems like over kill.
The case you will be getting is great though. Strong as an OX. You could always rebuild the machine, but with that model of Intellistation you will need to do some pretty extensive metal work to open up the I/O shield. It's a wierd set-up where the I/O shield is integrated with the PCI slots, not one that pops out with the motherboard.
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