|Adding more RAM will only increase performance if your system is lagging due to not having enough RAM. It's called the "law of diminishing returns". And it would have very little impact on gaming performance because gaming performance relies heavily on the graphics. If you have a crappy video card, it wouldn't matter if you have 2GB or 20GB RAM, you'd still have crappy game play.|
According to Crucial, your Compaq CQ40 has 2 memory slots & can accept up to 4GB of DDR2-800 per slot. You said you have 2GB, so it's probably 2 x 1GB. To increase your RAM amount AND maintain dual channel mode, you would have to replace both sticks or RAM with either a 4GB kit (2 x 2GB) or an 8GB kit (2 x 4GB).
A 4GB (2 x 2GB) kit sells for $98.
A single 4GB stick sells for $190.
An 8GB kit (2 x 4GB) sells for $380.
Newegg has the same RAM for $307. Is it worth it?