OK I have dell dimension desktop 3100 when I open my computer case and check it use DDR2 Pc 3200 I want to upgrade to 1GB so I go to this website Crucial I use their progarm to check what kind the memory I need they end up showing those memory are DDR2 PC 4200 so I dun no what to do ? Should I get them and install along with my original memory ? It that going to do any damage to my computer ? Just wondering

You can generally install faster memory along with slower memory (4200 vs. 3200) without incident, however you can attempt to avoid timing headaches and vendor nuances altogether by always using matching modules. Even then there can be issues if you're fully populating a motherboard to its maximum capacity. Usually, the best memory guide is the motherboard manufacturer's technical literature for your specific board revision. Some manufacturer's will test and certify various brands of modules to confirm stability.

Links:
Dell Dimension 3100/E310 Service Manual
DDR2 SDRAM - Wikipedia, the free encyclopedia