Yeah it's a remarkable system for its age surprisingly enough.
The box you had they did 2 variants of the motherboard, the general one most people ended up with board model ended in 02 and it had 6 resistors for voltage while the 03 has 11 which allows for twice the RAM with a BIOS upgrade.
I don't think I'd bother stuffing a card that cool in there, but you never know. It needs RAM more than anything right now, and I figure looking at some insanely well priced refurbs through ebay and newegg you can get a wicked i7 box with lots of solid parts and 8GB of RAM for upwards of just $300 ready to go if you have your own accessories and screen. I've got good newer accessories, and even that Radeon card or a newer 960 like you said could transfer over well.
It's crazy because in the 90s and earlier 00s it was normal if you did better intensity stuff to dump a PC for being slow after 2-3years at best. Now you can buy a badass quad core like I did 8-9 years ago, and as long as you throw a nicer video card in there and more RAM suddenly it's like semi-modern again and not sucking.