jlemi - Not sure what kind of your computer you have, but XP is very much capable of gigabit speeds. You need a computer with a gigabit ethernet card. So if the computer is a desktop that came with only a 100mb card, then purchasing a gig pci card should resolve that kind of problem.
If you have a notebook with a 100mb limit, that does complicate things. But I can assure you XP can handle gigabit ethernet.
The biggest reason to upgrade your computer at this point is due to XP being no longer supported by Microsoft. I'd be looking for a computer in which you can still get Windows 7 vs. 8. Even though Windows 7 is no longer sold, it is still supported.