The minimum system requirements for RollerCoaster Tycoon 3 are as follows:

Processor: Pentium® III 733 MHz Processor
RAM: 128 MB RAM
Hard Drive: 250 MB hard drive space
Video Card: 32MB video card (Such as nVidia GeForce2 or any ATI Radeon)

If you don't meet these specs you had better start saving for a new PC!