Hm
That sounds like a lot of headache job, moving proprietary Dell motherboard with proprietary connectors (and possibly proprietary motherboard screw locations) to another proprietary case. Have you thought about front panel connectors, or connecting USB 3.0 (Dell) to USB 2.0 (I hope, if not 1.0, since this model is from 2007), I don't know if you can connect USB 3.0 to USB 2.0, I doubt.
On the other side, you can upgrade your original configuration to GTX 1050 TI low profile, and you will need low profile brackets.
One card is advertised with brackets -
ZOTAC GeForce GTX 1050 Ti. Your power supply of 240 watt will support it. Or you can look
through reviews of all those cards, sometimes manufacturer doesn't advertise the presence of low profile brackets, so you might have more choices.
From my memory, dealing with small wattage power supply in Dells, I seen plenty conformations about 220 watt PSU supporting 75 watt cards in the past.
Is GTX 1050TI enough for you?