Right, Load up RCT2, load your park you want, then press ALT+Tab. This will let you our of the game (basically minimises it).
Next up, Load up 8 Cars, go to Rides >>> Edit Ride Stats (first one on the "Rides" menu).
A window will appear, in this select your ride from the drop down menu. Under the "Options" Frame in this window, there are a few text-entry boxes (Launch Speed, Lift Speed, #Trains, #Circuits, etc.)
Click in the lift speed box and type in your speed. Put it about 2mph above what you want it to be because it always trims a bit of when It sets it in-game.
You can use all the other menus to select car type (heh heh, wild mouse cars on looping coasters), track type, and operating mode (launched mine trains are good!) and other stuff...
Hope this helps!