Data Transfer With USB vs Firewire

I know my PC isn't Firewire capable. I'm not sure if it is USB 1.1 or 2.0. In order to fully appreciate a PC music server (i.e.: Logitech Duet) does one need to interface the external hard drive via Firewire (400 or 800), or will USB (1 or 2) suffice?

I'm trying to figure out if I can make my PC work without having to upgrade it to accommodate Firewire. Is it worth the cost to upgrade an older PC from USB to Firewire. What is the general consensus?
If you have an unused PCI slot at the back of your computer, you can purchase a PCI card with a USB 2 or a firewire interface. It's certainly cheaper than buying a new CPU. But if your computer is that old, in evaluating cost effectiveness, you might also want to factor in the other features that a newer computer will have relative to your current one.
Thanks Markphd.

I have determined that my PC has the USB 2.0. I guess I won't need to worry about a hard drive with firewire - would have been nice, but I think USB 2.0 will suffice.

I would appreciate recommendations for a decent external hard drive (500 Gig).