Actually, at this point I don't recommend using the X1950XT (nor would I recommend anything above a 9550 series) on such an old motherboard (P4T533C). Any CPU that's installed on that motherboard will seriously limit the performance of that card, as that motherboard does not officially support any of the 800MHz FSB CPU's (the FSB is officially limited to 533MHz, which is actually a quad-pumped 133MHz FSB). And RDRAM does not overclock particularly well. And its AGP speed is limited to 4x, whereas your other motherboard is faster (AGP 8x, official 800MHz FSB support).
And no, using an 0.8V AGP card in a 1.5V slot will not harm either card or slot; unless that card supports 1.5V signaling, the card will simply not work at all on that motherboard. On the other hand, using a 3.3V-only card in a 1.5V-only slot will result in a burned-out card and slot (assuming that the card physically fits into the slot).