To expand on Zach L's answer, we don't know you exact motherboard model (posting the model of your Dell computer would help), but based on the information provided, it's safe to say that you currently have a motherboard that supports DDR2 RAM modules (the individual sticks of RAM are called "DIMMs") and you are currently using PC2-5300 modules, based on your memory frequency.
Here's what you need to know to upgrade your RAM:
- You must buy DDR2 RAM
Your motherboard may not accept any other kind of RAM. Here's an explanation of why, for instance, first-gen DDR RAM won't work (from Wikipedia):
DDR2 DIMMs are not backward compatible with DDR DIMMs. The notch on DDR2 DIMMs is in a different position from DDR DIMMs, and the pin density is higher than DDR DIMMs in desktops. DDR2 is a 240-pin module, DDR is a 184-pin module. Notebooks have 200-pin modules for DDR and DDR2, however the notch on DDR2 modules is in a slightly different position than on DDR modules.
- You must buy paired/matching DIMMs that run at the same speed and install them in paired sockets, or buy a single DIMM and
Zach L and others have explained this already. You must also install your identical/paired RAM DIMMs in paired slots (if there are four slots, it will be apparent which ones are paired by how close they are to each other). It doesn't matter if you install your DIMMs in 1&2 or 3&4 on most motherboards, but for simplicity you can install them in 1&2.
Edit: As Hemmes points out below, there may be a scenario in which you buy a single DIMM of DDR2 RAM. In that case, you must install it in DIMM slot #1.
- You may want to get faster memory than you currently have installed, but only as fast as your system can use.
The memory bus on your computer can only utilize the full speed of memory up to PC2-6400. Buying faster RAM then that will not have any effect on system performance due to the limitations of your computer's Front Side Bus. I'd recommend you buy PC2-5300 DDR2 RAM (the kind you already have) or PC2-6400 DDR2 RAM (the fastest kind that your machine can use).