Add in textures that move down planes in eyespace for the raindrops themselves, and for the planes create a special mesh or array or whatever you prefer and spawn a random amount of splashes on triangles stored within it. Simple solution, and if I recall correctly this is, for the most part, how Splinter Cell does it. Wouldn't be suprised if MGS2 handled it the same way, but I haven't played it since my PS2 died (6 months ago).