It really depends on what you are doing and what the rest of your hardware is like.
If your CPU/mobo run at a speed that supports PC2100, then the RAM will downclock itself. This means you will see virtually no increase in performance (aside from the increase of capacity).
However, if your hardware supports PC3200, then you see a noticeable difference in performance.
Intel E6750...........PSN: ELBATROP
XFX nForce 650i Ultra
Patriot PC2-6400 8GB (4x2GB)
36GB WD Raptor
Samsung SyncMaster 931c
Samsung SyncMaster 750s
Windows 7 Home Premium 64