What version of Windows are you running on this system? If you are running a 32-bit version, your system can't really use more than 3.25GB of memory. Please provide the specifications of your system, as it is now.
I would suggest that 4GB (assuming that is what you have now) is all that you need and can actually use. Installing 8GB, even if the motherboard recognizes it properly, will not gain you anything in terms of increased performance. That is an older system and it really isn't capable of doing more than it is now.
I would further suggest you save you money and build/buy a new, modern system rather than spending money on this Dell.
Just because you can physically install the memory (as Crucial indicates) does not mean you will see an increase in performance. Many factors are involved, but the chipset used by the motherboard is only designed to use 4GB effectively. More than that is a waste of money and may even slow the system down slightly as the computer has to manage more memory than it was designed for.
Going from 4GB to 8GB on the Dimension 5100 will not increase performance. Sorry. Save your money for a new rig. That is the best advice here.