Looks like you've found the answer.
You can double check this by running CPU-Z on the dell and check the mobo info.
This will also tell you what CPU you are running so you can check if it's ok to use on the asus by looking supported CPUs list.
Then just take the heatsink of the asus and see what CPU it is. If it's a Intel Pentium 4 with HT, 670, 570, 660, 560, 550, 540, 530, 520 you can put in in the dell.
You can buy new budget LGA775 mobos from under £30.