PC2700 with an Athlon XP system is highly overated.
Yes, PC2700 will give you slightly better performance (not a huge increase), and more room for overclocking.
First try your current PC2100's capabilities (benchmarks, overclocking capabilities), then decide for yourself if an upgrade is worth the money (for a few % increase in performance).
I personnaly don't see the need for switching memory in your situation, unless you are starting from scratch...