You will want to make sure you do not overload your system with ram. Check your motherboard's specifications for max ram, and so long as you stay under the limitations, you should be ok. You will also want to make sure you purchase RAM with the same speeds as your current. For instance, if you have DDR2 PC6400 currently installed, you will want to make sure to purchase the same. The reason for this, is your system will function at the lowest possible transfer rates when it comes to memory. So one stick is PC-5300 will cause your remaining PC-6400 sticks to function at the slower, 5300 rate. To check your current RAM configuration, I would recommend taking a first hand look inside your tower, however you can also use a program like CPUID - System & hardware benchmark, monitoring, reporting
that will display your system information for you.