Unless you can get all the parts you want as pulls from retired servers, etc, you might want to rethink dumping too much money into that turkey. Cutting to the chase: a fully, totally, 100% tweaked out 8-core example of that machine might have roughly the total theoretical horsepower of a decent (but not amazing) quad-Core i7 but per-thread it's never going blow you to the back of the theater.
(One thing to consider about having 8 cores in that machine: when you have eight of them the share of memory bandwidth available to each core is actually somewhat less than a typical Pentium III machine, with higher latency thanks to the FB-DIMM technology.)
Personally I'd say, sure, go ahead and scavenge enough RAM to see if it'll go; with the addition of a video card the thing would make a decent Linux or Windows 7 box as-is, but I'd totally recommend against spending more than... not much, on it. It does suck that the original video card is gone. I looked into trying to flash a generic video card for a first-gen Mac Pro and pretty much what I came away with was the only third-party cards that have a compatible EFI bios floating around in the wild are as rare as hens' teeth. Linux or Windows will work okay with a PC video card stuffed in there, although it'll be sort of awkward as you'll loose the ability to access the "magic key combination" boot menus, etc, from the Mac firmware.
(One thing to consider about having 8 cores in that machine: when you have eight of them the share of memory bandwidth available to each core is actually somewhat less than a typical Pentium III machine, with higher latency thanks to the FB-DIMM technology.)
Personally I'd say, sure, go ahead and scavenge enough RAM to see if it'll go; with the addition of a video card the thing would make a decent Linux or Windows 7 box as-is, but I'd totally recommend against spending more than... not much, on it. It does suck that the original video card is gone. I looked into trying to flash a generic video card for a first-gen Mac Pro and pretty much what I came away with was the only third-party cards that have a compatible EFI bios floating around in the wild are as rare as hens' teeth. Linux or Windows will work okay with a PC video card stuffed in there, although it'll be sort of awkward as you'll loose the ability to access the "magic key combination" boot menus, etc, from the Mac firmware.