When using an NT-based version of Windows (i.e Windows NT/2000/XP), is it a good idea to enable 'System BIOS Cacheable' or similar setting that copies the BIOS into RAM? Please tell me all you know about this.
System BIOS on a modern machine (not on my IBM 286) is copied (or decompressed) into RAM anyway, so the question for System BIOS is only whether or not to make it cacheable. I would say "suck it and see".
Whether other BIOSes such as your video BIOS should be cacheable/ copied to RAM depends. For example, the old Future Domain SCSI controllers used memory-mapped I/O in the BIOS address range and copying that to RAM or caching it meant it stopped working altogether. In general I cache and shadow my video RAM , make my system BIOS cacheable and that's it.