The R9 270 and 270x cards use the Pitcairn (renamed Curacao) core, as long as your 7870 has that core it should work. If it has the Tahiti core then it probably won't work but may crossfire with the R9 280x which uses that core as does the 7950 and 70.
The Pitcairn 7870 has a stock core clock of 1000mhz, memory clock of 1200mhz and 1280 shaders and pipelines.
The Tahiti has a 975mhz core, a 1500mhz memory, and 1536 shaders and pipelines.
I've played around with a lot of hardware over the years and it's surprising what can work, but if you can't afford for it not to work buy the R9 280x, at least you know where you are at the time of purchase.