Goamins frae Scotland!

[gold_finger]

So what's the difference between UEFI and BIOS? Is it crucial to making a gaming PC? Does it make a difference to performance?

You'll have to read up on it yourself.  UEFI and BIOS are just the types of firmware that computer uses to do hardware checks and initialization when you first power on the computer.  Then they point to a part of the hard drive that holds boot loader(s) which then take over rest of boot process to get into an OS.  Has nothing to do with whether or not you will be gaming with the computer.  Here are some links to more info:

https://en.wikipedia.org/wiki/Uefi
http://www.rodsbooks.com/linux-uefi/
https://www.happyassassin.net/2014/01/25...work-then/
http://www.ilovefreesoftware.com/17/tuto...-uefi.html

I was planning on installing my modern Windows OS for newer games that don't work on WINE, then install Linux Lite (or Mint) to use as my main/everyday OS, and maybe install AROS for Amiga games, OSx86 for Mac programs/games, and FreeBSD, basically, every OS type available! I was thinking of also installing legacy Windows, such as ME and XP for games that don't work on Windows 7/8/9 or WINE. Do you think all this would be possible to do with one machine?

I have no knowledge of AROS, so have no idea what the requirements for it are.

Can you put them all on same machine?  Probably.  If installing legacy Windows like XP, you must use BIOS/Legacy mode and have the hard drive partitioned using traditional MBR (msdos) partitions.  With possible exception of MAC OS (and AROS?), all others can be installed in BIOS/Legacy mode too.  I have no experience with MACs but think they may require EFI mode.  A possible way around the problem is to have more than one drive and install the MAC OS on a different drive in EFI mode if that's necessary.  What you'll then have to do is change the boot mode to EFI and point to the MAC drive for booting when you want to boot into it.