I can't remember if it's the Dell SFF machines, or HP Slimlines . . or maybe both . . that have their own compact size that is different from TFX.
I would suggest taking careful measurements of the PSU. If it's TFX size, and it has the same type of MB connector, then you can get a TFX power supply.
If it's the same size, but the Dell uses the 8-pin MB connector instead of 24-pin, then I think there's an adapter available for that.
NOTE: please pay attention to the new PSU's number of amps on the 12V rail. 12 x the number of Amps on the 12V rail gives you a true indication of how many watts the new PSU is, rather than just the total wattage rating.
For example, the original PSU shows:
+12V --- / 17A
12V x 17A = 204W...