[Thank The Cheese]

My sister has an eMac with USB 1 and syncs her iPod video fine. It takes a ridiculous amount of time to do the initial sync (hours, depending on how big your iTunes library is), but after that it doesn't take very long, as it's just updating the songs or Podcasts added since the last time it was connected. Unless you're adding gigabytes of stuff to your iTunes on a daily basis, USB 1 will be adequate.

If it's an old mac it may not be worth it (or even possible) to install a USB 2 card. as icemanjc suggests, before you buy anything post your system specs -- would hate for you to buy something you can't use.