Well, I would simply do the following:

A) save current position in a temporary vector
B) calculate new position according to vel_x and time_step
C) use temporary vector and new x,y,z position to trace
D) if you hit something, place particle to the hit x,y,z position and set velocity parameters to the bounce direction

Nevertheless, I could imagine that if you do this for each particle, your game could get awfully slow.

PhysX has particle fluids, though, this is not yet ported to Gamestudio. So, if you need a high performance and physically correct solution, try that.