On an AMD 2600 (2GHZ), Radeon 9800 Pro (128 MB video memory, 1GB RAM), I can have huge numbers of large particles and they do not seem to make much dent in a target 60 FPS.

On an AMD 950, Nvidia TNT 2 (16 MB video memory, 384MB RAM), the same particle fountain will cut that 60 FPS to between 20 and 30 FPS.

If you have a good graphics card, maybe the function you are using for each particle is too complex?