Look, the Rampage vi Apex is one High end board, and you want to use a mid end Kaby lake CPU (7740x), the limitation of processor are that it does not have Turbo Boost Max 3.0, AVX-512 support and also it has only 16 PCIe lanes support, saying this it means that if you have 8 ram slot board you'll be able to use only four, so as ApEX have 4 RAM slots you will have no problem but you'll gonna have some limitation, so if you have some spare $ (50-60) i suggest you to go with 6 core i7 7800x. With it you'll have no issue with the RAM, and only limitation will be its 28 PCIe lanes support, it means if you have 2 GPUs one will be PCIE 3.0 x16 and Second GPU will be PCIE 3.0 x8, and you'll have additional PCIE 3.0 x4 free for PCIE NVME SSD. The Decision is yours. I can strongly suggest you (obviously if you have that amopunt of $$$), to go with i7 7900x CPU with the APEX board, in that case you'll have no issue at all (you'll have the all 44 PCIE lanes Support which means you can use 2x SLI GPU PCIE3.0 x16 and still have 12 PCIE lanes free for 3x PCIE NVME SSD or 1 PCIE NVME SSD + 3rd GPU PCIE 3.0 x8).
So I think you have now enough info to make your decision.
PS. If you'll use your PC only for gaming and some light works I can also suggest you to go with cheaper option of 8700K + Z370 Board:
1.
https://www.amazon.com/Intel-BX80684...keywords=8700k
2.
https://www.amazon.com/PRIME-LGA1151...ords=z370+asus
the PSU, RAM, SSD and GPU stays the same as for APEX + 7740x