Should I Choose an mATX or ATX Motherboard for This LGA1700 Build?

The main difference is size and expansion. ATX boards are larger and usually provide more PCIe slots, while mATX boards are smaller and often cheaper. For a normal build with one graphics card, one or two M.2 drives, and no extra expansion cards, mATX should be completely sufficient.

Check the case specifications, though. An ATX case normally supports ATX, mATX, and ITX boards, while an mATX case cannot fit a full-size ATX board. There is no special building process for mATX; the installation is essentially the same, just with less room around the components.

Full ATX is mostly worthwhile if you expect to add several expansion cards, multiple storage drives, or a custom water-cooling setup. Otherwise, it tends to add size and cost without improving gaming performance. The i5-12400 does not require an expensive motherboard, so a well-equipped B660 or B760 DDR4 board at a reasonable price is enough. Make sure the specific board supports the CPU with its installed BIOS version, although most newer boards should already be updated.

I’d lean toward a decent B760 mATX DDR4 board. Something like the Prime B760M-A WiFi D4 is suitable for an i5-12400 and should have adequate VRM cooling, memory support, and storage connectivity. I’d avoid the very cheapest boards if they have minimal VRM heatsinks or weak power delivery.

The main trade-offs are fewer PCIe slots and potentially fewer M.2 or SATA connectors. If she only needs the GPU and one or two SSDs, those limitations probably won’t matter. mATX also lets you use a smaller, easier-to-move case.