AMD just announced its new R9 280X earlier this week, so it's unlikely you've gotten your hands on one of these cards just yet, but when you do, you might like to know that this card is compatible with AMD's Radeon HD 7900 series in CrossFire. As our own Chris Angelini reported in his review, the cards will work in tandem if choose to go that route:
"To answer whether existing Radeon HD 7970 cards can be paired with the new R9 280X, yes, they work together," Chris writes. "A quick Fraps-based test showed one 280X hitting 52.9 FPS in Battlefield 3 at 2560x1440. Dropping a 7970 GHz Edition next to the newer board pushed frame rates to 102.3 FPS. When it wasn't in use, the 7970 properly spun down according to AMD's ZeroCore technology."
In case you missed it when these cards, launched, the R9 280X’s specs are very similar to the Radeon HD 7970 Ghz Edition. The Tahiti GPU in the R9 280X features the same 2048:128:32 configuration, and packs a 384-bit memory bus rocking 3 GB of GDDR5 (which AMD says should operate at 6 GT/s). The one notable difference is the "new" card's engine clock, which tops out at 1 GHz. Other than that, these cards are a great match.
The folks at HardwareCanucks came to the same conclusion and provide visual proof in the form of the screen cap featured above.