The R9 280X has a 100 MHz higher core clock speed than the R9 290X, but the R9 290X has 224 more Texture Mapping Units than the R9 280X. As a result, the R9 290X exhibits a 211.2 GTexel/s better Texture Fill Rate than the R9 280X. This still holds weight but shader performance is generally more relevant, particularly since both of these GPUs support at least DirectX 10.
The R9 280X has a 100 MHz higher core clock speed than the R9 290X, but the R9 290X has 96 more Render Output Units than the R9 280X. As a result, the R9 290X exhibits a 92.8 GPixel/s better Pixel Fill Rate than the R9 280X. However, both GPUs support DirectX 9 or above, and pixeling performance is only really relevant when comparing older cards.
The R9 290X was released less than a year after the R9 280X, and so they are likely to have similar driver support for optimizing performance when running the latest games.
Both GPUs exhibit very powerful performance, so it probably isn't worth upgrading from one to the other, as both are capable of running even the most demanding games at the highest settings.
The R9 290X has 5120 MB more video memory than the R9 280X, so is likely to be much better at displaying game textures at higher resolutions. This is supported by the fact that the R9 290X also has superior memory performance overall.
The R9 290X has 332.8 GB/sec greater memory bandwidth than the R9 280X, which means that the memory performance of the R9 290X is massively better than the R9 280X.
The Radeon R9 290X Crossfire has 5632 Shader Processing Units and the Radeon R9 280X Sapphire Toxic 3GB OC Edition has 2048. However, the actual shader performance of the R9 280X is 1331.2. The R9 290X having 4300.8 better shader performance and an altogether better performance when taking into account other relevant data means that the R9 290X delivers a massively smoother and more efficient experience when processing graphical data than the R9 280X.
The Radeon R9 290X Crossfire requires 500 Watts to run and the Radeon R9 280X Sapphire Toxic 3GB OC Edition requires 250 Watts. We would recommend a PSU with at least 1250 Watts for the R9 290X. The R9 290X requires 250 Watts more than the R9 280X to run. The difference is significant enough that the R9 290X may have an adverse affect on your yearly electricity bills in comparison to the R9 280X.