ASUS PRIME H310M-E R2.0 specs here
mATX Form Factor
8.9 inch x 7.3 inch ( 22.6 cm x 18.5 cm )
Honest answer is that you would be better to wait until you have sufficient funds available to replace not only the case but the PSU, the Dell Vostro 400 is dated technology and back in those days they used propriety components so chances are that the stand offs inside the case will not be compatible with any modern MB.
Not mentioned above, GPU, RAM, storage device or an operating system such as Windows 10, you may already have a SATA storage device that you can use but the least expensive good quality RAM and Windows 10 will cost a minimum of $135 US, an example list of parts here
and based on your proposed CPU and MB, list is minus any GPU as allowed for you using the onboard and no storage device as you may already have one.