Looks to me that the power supply may be under powered. I see specs of 275 watts, you need a quality 400 watt. You may have to buy a power supply but be prepared that the aftermarket psu that you buy may not fit that case. Dell is notorious for making their cases fit proprietary power supplies. You need a quality 400 watt psu with a 6 pin dongle.
You'll need to find a proper PSU for it, and in a dell SFF that might be tricky. To be honest, trying to find a PSU in a TFX or other SFF format is not an easy thing to do. Now, that being said, a GTX 650 has about half the firestrike score of a GTX 750ti.
AND a GTX 750ti -usually- doesn't require a power connector, it draws under 75w or so. That means it can usually be run off a 250w PSU without issue. And it's faster than the GTX 650 by quite a lot. So I'd ditch the 650 and go to the 750.
Generall the GTX750Ti low profile is the go to card for these. A GTX1050 would be better if it doesn't require the 6 pin PCIe power cable that ytou can't get due to your PSU situation.
Check with Dell.com support and download the Service manual. Read the specs. on the PCIe slot. You will be looking for any reference to a 35W limit there. In that case GT730 will be your best bet.