Just make sure you get the exact same card (to make sure that the SLI works) and also be sure that your motherboard support SLI (recommended ar 8x/8x, 16x/4x is fine, but will decrease performance)
You don't need the same "exact" card to SLI, just the same card. There are no 16x/4x SLI boards. Nvidia requires both slots to be x8 for certification.
@OP: SLIed GTX 460s easily beat a single HD6950 and especially so if they have a high OC like you should have with the MSI Hawk.