1) I was never quite sure of this, but it according to Wikipedia
, "microATX was explicitly designed to be backward-compatible with ATX. The mounting points of microATX motherboards are a subset of those used on full-size ATX boards, and the I/O panel is identical." Maybe since the source is Wikipedia, I still shouldn't be sure of it.
2) Double-check the manual once you've decided on a motherboard.
Are you comfortable that old case provides enough space and airflow for the new CPU and GPU (and their coolers)? Does it have exhaust fans besides the one on the PSU?