Depends on many factors. I do not know much about those Dells, but I am pretty sure that they use the same heat sync on the I5 2400 and I7 2600 (locked) models, so I would bet you are fine.
I would try the current heat sync and fresh thermal paste with the I7.
Load the CPU with prime 95 blend. If temps are under 80-85c, the heatsync is adequate.
If temps are higher than 85c or the cooler is noisy, you could throw on a cheap tower cooler like a H212.