A few things to consider when upgrading,
Will my power supple handle the increase? If you have alot of hardware connected to your power supply you may need to upgrade it. Usually a 350 to 400w power supply will be adequate.
What operating system are you running? If your OS is Windows 2000 and above you won't need to make any changes but if you are running Windows 98 or ME then if you install more then 512MBs you need to make changes to your system.ini file. If you are not running Windows at all then you need not be worried about making any changes.
Compatibility can be an issue so make sure you have similar types of memory.
Is my BIOS up to date? Sometimes motherboards need to have the BIOS updated to handle different types of memory. The industry changes so often that memory densities can cause the system to act weird so I would always recommend doing a BIOS update before increasing the memory. Sure, this may sound like overkill but past experience has taught me alot and if you have not done a BIOS update before I would first install the new module and test the system before ultimately doing the BIOS update.
Woops almost forgot,
Make sure and match as much of the characteristics of the old memory you can with the new one. You may not be able to find the exact memory you currently have but get as close as you can. Try to match: speed, Cas Latency, and if you are running dual channel the chip count on the module.
Just a few of my thoughts.
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