It's not that it's obsolete because it offers "better" cooling than atx but it's rare and you don't find any aftermarket cases anymore since lga 775 days. It might as well be handled like a proprietary form factor. Last I saw it being used was a lot with core 2, then a rare ivy bridge model but I haven't seen them use it anymore.
The t3500 is a modified atx. It's also flipped. It will fit in a atx case. Just the cpu and ram was moved. The io and expansion slots are still at the same place to fit atx. The issue you'll run into is dell uses a proprietary front panel connector. I'd just stick with the case. If you need to upgrade, it should be a whole new build.