Square has released the specifications for the PC version of Final Fantasy XI. The game obviously requires Windows 98/98SE/2000/XP with Japanese language support and Direct X 8.1. On the hardware side, the minimum requirements are the following: Pentium III 800MHz and higher, at least 128MB RAM, a graphics accelerator equipped with an nVIDIA GeForce series chip, a minimum of 4GB free space on the harddrisk drive, a CD-ROM drive and (last but not least), extremely surprising for a MMORPG, a 56kbps modem or faster internet connection.
Square's recommended configuration looks like this: Pentium IV and above, 128MB or more RAM, a graphics accelerator with a GeForce3 (or higher) graphics chip and at least 64MB video RAM, 4.5GB of free disk space and a 32x speed CD-ROM drive.
Since you will have trouble working with Windows without a keyboard and mouse, it is probably not necessary to mention that those two devices are required to get the game running on your PC as well.
The PlayStation 2 version of Final Fantasy XI was released on May 16th in Japan, with a US release to follow in early 2003. The Japanese PC version is expected to hit shelves sometime this fall.
Thanks to Paul Kangas for the tip.