See Chips and Modules section.
First of all...make sure that you looked at the right model at Crucial. I see that there is also a 5100C model for Dell Dimension...you need to make sure that you don't have that model...or that both models take the same memory. I just checked, both take the same memory.
Different types are different speeds, with the system always running at the lowest speed which is installed.
DDR2 PC-4200 is the slowest module which your system supports, while PC-8500 is the fastest.
DDR2 is dirt cheap these days. If I were buying I would just buy two 2GB modules of whatever happens to be cheaper (from those categories listed) on that day, at that moment. The 4GB package of PC-4200 for $67 is what I would go with, regardless of operating system now or current system configuration.