3DMark
3DMark is the latest version of this hugely-popular synthetic benchmark. Making use of DX11 features such as tessellation, compute shaders and multi-threading, it provides modern-day results and is available as a free download.
Knowledgeable readers will know that R9 390X is effectively an overclocked R9 290X. Sapphire's minor overclock aside, 390X is approximately 10 per cent faster than the previous single-GPU Radeon champ first released 18 months ago.