I have the xfx 7870. Works really well. Ditto what forbin said about checking your PSU and also the actual card itself is quite big, dwarfed the card it was replacing sizewise.
If you can find one, you also have the HD7950 3GB which is around the same price and would offer a little more performances.. I just bought one for 189euro including a 3 games voucher. GL!
I'm not that experienced in hardwares as others but I bought the HD 7870 2 GB GhZ Edition few months ago and it runs pretty well here. Enough performance for a good amount of money imo.
I compared the SAPPHIRE DUAL-X R9 270X 2GB OC with XFX HD7870 Double Dissipation (FX-787A-CDFC) and the main difference is the memory clock. the R9 is 800 mhz faster. For the rest there isn't much difference. amount of stream processors in the xfx is 1536 over 1280 of the R9. XFX uses 10 watts more.
So it's almost the same and more of a "what feels better" choice.
I know a lot about this things and i recomend hd 7870 its better especially xfx.
R7 is in a range with gtx 650 1gb which im planing to buy but if you have more money buy the xfx radeon.