Black & White 1 - A "God" game kind of RTS. (Only problem is that it's VERY hard to find these days, maybe on Amazon or eBay).
Windows 95/Mac OS 8.6, Pentium II 350MHz/G3 333MHz, 64MB RAM (96 on Mac), 8MB video RAM (4MB will suffice & "on board" will work, slow), 8X CD-ROM, 600MB (750 with Direct X & save)
Rollercoaster Tycoon 1 - The start of a great generation of theme park simulating. (Probably hard to find too).
Intel Pentium 90MHz CPU, 16MB RAM, 55MB hard disk space, 1MB GPU
Rollercoaster Tycoon 2 - The sequel to the above, more rides and more fun. (More likely to find this, but not guaranteed to be easy).
Intel Pentium II 300 MHz CPU 64MB RAM
Just out of curiosity what are the specs of it besides a 600 MHz CPU?