SSDs don't do anything really for game performance (just reduce load times and general PC boot times, as games are generally very low on the I/O). Instead of getting a bigger SSD, I'd instead suggest getting a large HDD and using the SSD just for windows (OS on the SSD is where 95% of the improvements are found) and for open-world and heavily modded games (specifically ones with large texture mods, as those are the most I/O heavy).

As for what GPU will fit: you'll need to crack open your case and get a measuring tape for that one. All GPUs will list their dimensions in the specs, so you can see that for yourself. The measurements inside your specific case will depend on a wide variety of factors including any dell customizations you did prior to buying and any additions and changes you made since buying. As such no one can tell you if a card will fit except you, unfortunately. It's really quite simple to find out, at least.