If it’s a video camera, I’d be surprised if it DIDN’T require FireWire (or IEEE 1394, same thing) – this is pretty much the industry standard, and it’s what FireWire was designed for.
As for cards, I don’t think you can go wrong; XP and FireWire are roughly the same age, any FireWire card will have XP support.
If you have any other upgrades to do at the same time, though, look around - FireWire ports are starting to be pretty common on other sorts of cards - particularly sound cards, for some reason, but also networking and video cards. It apparently costs very little to add it. Also, many recent motherboards have intrinsic firewire support, so you may already have it, and Dell just didn’t hook up an external port for it.
If the machine’s a little older, I’d look at getting a USB 2 + FireWire card; it will cost about the same as the plain FireWire, and lots of peripherals are starting to come in USB 2 (roughly the same speed as FireWire) versions now.
If you’re planning to hook an iPod to it, look for a card that’s labeled as providing power on the port - it’s an Apple “standard” but somewhat unusual on the Windows side - this allows the iPod to charge from the port without being otherwise plugged in to power, nice if you can get it.