It’s worth it to clean all those drives out. They are probably from generations past and can benefit from a modern replacement. And do you use the optical drives that much so you need two ?
If you upgrade you’re pretty much going to have to clean out - very few newer motherboards support IDE at all and pretty much none support more than a single IDE cable supporting 2 devices. As far as drive bays, SSDs are usually 2.5", but you can easily get a bracket to convert them to fit into a 3.5" drive.
The better new cases are so much better than the older ones I used to have - larger fans mean more air with less noise and lower temperatures for your CPU & GPU. I personally have two Antec 300s and a Corsair 500R, and even the Antec 300 is miles better than the cases I used to get.
One drive is a Blu-Ray and the other is a DVD recorder so I do need two unless I buy a replacement optical drive. The IDE drives can go into an enclosure and become external drives if it came down to it.
I’ll look into the Antec cases