With computers the biggest danger when opening them up is static electricity. Just make sure to touch the side of the power supply (the big metal thing at the top of the case where the power cord is plugged in) while the cord is still plugged into the wall before touching anything else. After this, you should unplug the power cord. This will remove any static that you may have built up. It also helps if you have a room that isn't carpeted to do the installation in (ever shuffle around on carpet in socks, and then sneak up behind a sibling when you were a kid?). I think the Dell manuals talk about this, but can't remember right now.
PCI cards do need a little bit of force to push in, so don't worry when if it doesn't just slide in. As long as you are pushing down evenly on both ends of the card, you will be fine.
Have you ever cleaned out your computer? If not, now would be a good time to use a can of compressed air to blow out all of the dust. Especially the dust on the processor and in the power supply.
Good luck!
goates