There are no guarantees,
SP3 in general has caused fewer issues than SP2 did, it is ofcourse recommended that you back up before doing anything like installing a service pack (but the same goes for any other hardware or software).
Worst case scenario you perform a PCRestore after the fact and then upgrade. I have done it countless times on countless systems without incident, and have had no issues except Automatic updates failing to work after the install (very easy fix, post here if you run into it).
Some best practices before installing Service Pack.
1. Update BIOS and Drivers at support.dell.com
2. Back Up Data
3. Update and temporarily disable Anti-Spyware real-time scanner (can sometimes block the installation of core files) (Note: Anti-Spyware, not Anti-Virus).
4. Make some coffee, this can take a while, be patient, even if it is taking an hour.
The additional security especially with wireless, and performance gains most users experience is definitely worth it.
I hope this helps.
I will follow this thread in case you need more help, I love participating and helping out, but am opposed to this CNET/Dell alliance as you may have noticed. So will not be active here in other posts.