I am unsure if this secondary PCI-E slot is for SLI only
or would work with another card running together.
This is one part where you do not have to worry. There are not PCI-e connectors 'just for SLI'.
PCI-e is not even designed just foir graphical cards. Like it predecessors PCI and PCI-X it is meant for generic cards. That can be a graphical card, but also a RAID card, a serial port, a modem, a wireless card, a regular NIC, or just about anything else.
The only problem you might run into is drivers. Some older operating systems (e.g. windows XP) only support one graphical driver. If you have modern OS then this should not be a problem (e.g. Vista or newer, any or the Linux distributions, BSD's, OS X, ...)
Before you buy a new card I would:
- make sure that there are up to date drivers which work fine with other cards.
- Check if it needs 6-pin or 8-pin power plugs and if you have a PSU which can deliver that power.
PS: Please add the OS. You mentioned gaming so I assumed windows. But that is a guess on my part. And increasingly weak guess now that gaming on OS X and on Linux is gaining.