Mandrake 9.1 was a nice OS, but if you have very modern hardware, you want to choose the newest distro you can. Mandrake 9.1 uses a 2.4 kernel, which is rock solid; tried and true. But kernel 2.4 had problems with some AGP types, and also uses a depreciated sound system by default. Mandrake 10 uses kernel 2.6.3, which is very modern, fast and sleek. It no longer uses the old sound system, and has support for all major AGP types, SATA, and 64-bit Athlons (among many, many other things). I really like kernel 2.6.x, I think it's a step in the right direction, so I recommend Mandrake 10.
P.S. - Any operating system that you run directly from CD rather than HDD will be slow. Distros that run from CD do have their purposes, but you really cannot use them to learn Linux, because you cannot write to the system and make changes. Besides, if you really want to use an operating system, it's *horrible* to run it from a CD! Just get a real install version like Mandrake 10, and use "Live" type distros to show your friends Linux when they're too afraid to change their hard drive. :)