Have you looked inside your computer to see how many sticks you have now? How many free slots do you have? If you only have one stick now, then they don't have to be installed in pairs.
http://www.crucial.com/store/listpar...3000&submit=Go
Crucial will guarantee that the ram they deliver will work in your computer, which is nice. Dell will tell you what to buy too, so you can just ask them, if you are unsure.
As far as how much, it really depends on what programs you run and how many of them at the same time. Take a look at your task manager (right click the taskbar and select task manager) to see how much ram you are using under normal load. If it is close to 512mb, then I recommend getting more. In fact, I would probably go ahead and get as much as you can afford, up to 1gb, because it will be cheaper to buy it all at once, and you can never have too much ram. The minimum I like to use is 1gb, but 2gb is always nice. Of course I usually have 30 applications or so running at the same time.
The basic idea behind ram is that you never want to run out. When you run out of memory, the computer uses temp space on your hard drive for memory, and it is much much slower than ram, so it drastically slows down your computer.
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