Your Compaq system has 2 slots.
Each slot can hold up to 1 gB of DDR2 PC2-5300
The system can hold a maximum of 2 gB of DDR2 PC2-5300
You currently have 1 gB of RAM - you must physically look at the slots to see what you have installed.
Most places will charge you $50 to install the RAM - and that's a lot for:
1) Opening the case
2) Removing the old RAM
3) Inserting the new RAM
4) Closing the case
5) Testing by booting the system up
Post back if you'd like instructions on how to do this.