You can just install more RAM as long as you have four slots right now (some motherboards only have two, particularly in laptops). Note that you'll want to make sure that you insert the RAM so that it will bank properly. The two 512 sticks (I assume) in there right now should be in the correct places to bank with eachother, so just put the two new sticks in the two other slots (might be next to eachother or might alternate). As long as the RAM are in the correct slots they will dual-channel properly (DDR2 banks in to sets of two, so the sets of two just have to match).
If you only have two slots, then definitely just take out the existing RAM and replace it entirely.
You'll need to make sure you get the right kind of RAM. The RAM needs to be the right generation (DDR, DDR2, DDR3) and as fast as or faster than your current RAM to avoid a loss of performance. It's probably DDR2, look for a speed number that looks like "DDR2 666" or similar. You'll need DDR2 with the same or higher speed number.
No configuration will be necessary, it will work just fine out of the box. You might want to look in BIOS to confirm that the new RAM count is correct, to make sure that it's installed properly faster than the time it takes to boot your OS.