I will keep the chassis, you never know when you might need one.
Yes, as long as the PSU is up to it. Dell (and HP / Compaq) tend to have unique fit components that makes it difficult to replace or upgrade - that goes particularly for motherboards and PSUs.
My current PC case (Cooler Master mid-tower) is almost 11 years old now - it's on its 3rd mobo, processor and GPU, 2nd PSU, now with SATA SSDs & DVD writers - in fact the case and sound card are the only things that are from my original build. The 3.5" floppy drive finally got replaced with a 2 x USB 3.0 front panel to access the USB ports on the mobo.
I find that incremental upgrades are the best way for me to keep reasonably up to date hardware-wise.
SD