Refer to the documentation for hardware requirements. It really depends on what you want to do. But I think the lowest requirement is a 90mhz processor with 64mb of ram and a 1g drive. I have had it running very happily on a pentium 200 with 64mb of ram and a 2g drive.
As for forum software it really depends again on what you want. You can install Xoops or E-Xoops in an Ibay and run the forums in them. That is what this forum is using.
Tony