Upgrade GPU question for Area 51 R2

Hi,

I recently purchased the Area 51 R2 with a 970 GTX. I am looking to buy a 2nd 970 GTX so that I  can run a SLI configuration. I have never run an SLI configuration before and was hoping you guys could assist me with this.

Is there a recommendation on what the 2nd card should be? I am not sure what Dell uses and if I need to use the same or if I can buy a 2nd 970 GTX elsewhere and SLI them together.

Help would be greatly appreciated.

Thanks.