There are no benchmarks for the Intel HD 510 at this point in time. The following link to NotebookCheck.net ranks it close to the performance of the Radeon HD 8470D graphics core that is in the A6-6400k. However, that is based on specs alone, there are not benchmarks for it. The middle column where it states "Class 1" rank mostly mobile graphic cores and a few desktop GPUs. You can find the Radeon HD 8470D and the Intel HD 510 both in the lower half of "Class 3".
http://www.notebookcheck.net/Intel-HD-Graphics-510.149943.0.html
In terms of overall processor performance, the Pentium G4400 (3.0GHz) would definitely be more powerful than the A6-6400k (3.9GHz).
- If you plan on installing a graphics card in the future, then get the G4400.
- If you plan on upgrading the APU / CPU for better performance in the future, then it's a toss up. The best graphics core in an AMD APU is the Radeon R7 (Kaveri) found in the A10-7850k and it is a bit more powerful than the Intel HD 530 graphics core which can be found in a CPU like the Core i5-6400. While the a Skylake generation Core i5 CPU will have much better processor performance compared to the A10, it will also be more expensive; $190 vs $85 currently in the US. In the link provided above, both of these graphics core can be found towards the bottom of "Class 2". If you add a graphics card, then the Core i5 will definitely outperform the A10 in games... but that means more money.
- If you want the best gaming performance now and don't want to even bother thinking about the future then buy the A6-6400k.