|You didn't say whether your computer is a desktop or a laptop, but it appears it must be a desktop - a Dimension 3100. |
Apparently you have two ram slots, your mboard cannot support recognizing larger than 1 gb ram modules, so you must have 2 - 1 gb (1024 mb) modules, a total amount of 2gb ( 2048 mb), and you can't install more ram than that.
64mb of the ram is presently being used by (shared with) your onboard video, but your figure of 1982 mb probably isn't right - it would probably be 1983 or 1984, not 1982, depending on whether or not the bios subtracts 1 mb for the conventional memory amount the operating system and the user can't use for data.
The ram you can install - e.g. Kingston
Kingston, and most other ram module makers, tell(s) you how many ram slots you have.
Kingston's info says your mboard has the Intel 915GV main chipset. When you search for ram for a brand name system, the module model numbers are often specific to the brand, but any ram specified for any mboard maker's model with a 915GV chipset should also work fine.
There is also info regarding the ram in your Dell Owner's manual for the model.
In your mboard's bios Setup you can find out at what speed the ram is running at.
The Service Tag number is probably shown in the bios Setup - it's probably also on, or at least on, a label on the outside of the computer case, probably at the back. You can use that on the Dell site to find the exact support info about your model.