Yeah I remember flashing an old AGP slot Radeon 9800 pro to an XT, since they used the same chipset but were priced differently, it really increased performance for free.You can mess around with the BIOS of the card but you need to know what you are doing. Of course, if it's already "bricked" then how much harm can be done huh? Lot's of good info on this here.
We'll need to know what PSU you have before recommending a video card upgrade.
My PSU is a Corsair TX750