If u want to be on the safe side get exactly the same card again. Most manufactorers pimp their cards in one or the other way. While it can work with different cards, i wouldnt risk it. Also, it is pointless to use SLI without a bridge. Your SLI-Motherboard should have had one delivered with it anyway.
If u get different cards make sure they have the same amount of VRam (mandatory!) and try to find out of they have tools that allow u to adjust the MHz-numbers, etc so u can edit them to make them equal. Otherwise u might risk system instability.
Also, make sure u check your power supply and cooling. Keep in mind that 2 GPUs produce MORE than double the heat because every GPU heats up the rest of the system as well.
http://www.realhardtechx.com/index_archivos/Page362.htm <- This link provides u with a chart where u can see what kind of supply u will need. The chart is conservatie, so having what they specifie is mostly enough, especially if u dont overclock your system.
I got a Corsair AX860i Series PSU and it works like a charm with my system (see signature below).
Another thing about SLI:
Be prepared that it wont always work. I am currently playing World of Warships which almost doesnt work at all with SLI-enabled. So u might run into problems with specific programms. As WoWs is still in Beta i dont mind it much, still: it is a brand new game and there is no gurantee that they will ever add support for it. Same with a lot of older games and programms, not all can handle SLI-Configs.
