The ATX standard has changed very little in the last 15 years, which means some rather old computer cases can still accept new motherboards, as long as you also replace the power supply.
The bad news, as I stare at the case that once housed a Micron Client Pro 766 Xi (a 266 MHz Pentium II that was state of the art in 1997) is that front-mount USB ports were unheard of in those days, as were digital camera memory cards. Instead, machines of that era used obsolete floppy and Zip disks for removable storage. They also typically had more 5.25″ bays than we need today. When CD burners cost $400, most of us kept a reader in as well, to avoid wearing out expensive burners prematurely.
The cheapest way to modernize these old cases–which often tend to be high quality, and free for the asking–is to replace the floppy and/or Zip drive with a $5 memory card/USB reader that fits in the 3.5″ bay or a costlier 5.25″ model. Internally, they plug into a USB header on the motherboard the same way a modern case’s front-mounted card readers and USB ports would connect.
If you have unused slots in the back of the case, you can get a 4-port USB bracket or 2-port bracket for those as well, which plugs into two more USB headers on the motherboard. For that matter, eSATA brackets are cheap too.
I haven’t bought a new computer case in more than 10 years, and that’s my trick. Some 20th-century cases look rather dated today, but I don’t mind. Black peripherals in beige cases are uncommon today, but that was standard issue in the PC/XT days, so I don’t mind that either.