Check out this web site:
http://www.off-road.com/jeep/cherokee/98/06_jun/cooling/cooling1.html
I did this exact conversion along with running my aux fan from a switch, and I can say this combo totally wiped out my overheating problems. I live in Southern California, and the heat here has always been a problem for my XJ. With this upgrade(and prior to my radiator and t-stat upgrade), I went from averaging 210 F and dropped to 190 F.
I just put in a GDI 3 row radiator and 160 thermostat about two weeks ago. With this upgrade, I don't have to run my aux. fan any more to stay at 190 F...although I did tun a little test using it. With the two fans running, and with the new radiator and t-stat installed, my XJ does not get over 183 F... even after letting the jeep sit and idle in 103 F temperature for 20 minutes. This was a thing I thought I would never experience in this vehicle.
Another thing I noticed with the fan upgrade was a power increase. The "in the seat" sensation I experienced was actually similar to when I installed Rusty's K&N air tube. Removing the clutch fan removed a 5 lb weight the motor no longer had to spin around, increasing motor power efficiency. As for the radiator and t-stat upgrade, I noticed an overall increase in performance. When my temperature was at or over 210 F, it seemed as though throttle response became sluggish, and climbing hills on the freeway became a chore. Now, since it never even sees the middle mark of the temp. guage, I feel the old XJ (262,000+ miles) is showing its appreciation through better performance.
I hope this helps a bit. If you have any questions about this upgrade, I can answer them. I can also put up pics for anyone interested in seeing this upgrade. Definately check out the link I put up though...good stuff.
An idea to consider if you do this upgrade would be to connect not only the aux. fan to a switch, but also the newly installed main fan. This way, you will have total control over when they spin, and for how long they spin.