I'm have been looking around at a few sound cards and some of them say they need to be put into a PCI 2.1 slot. How do I know if my computer has those type?
If your computer is newer than probably 1997 or so (PCI 2.1 was released in 95) it will have 2.1 or a newer version of PCI. It should say in the manual as well, but I think you are safe in assuming the cards will work.
