Ok, first things first:
What made you think you need more RAM?
I don't want to rain on your parade but that pc is 7 or 8 years old. There's a lot more to CPUs than cores and clockrate, but I won't go into that. What I'm trying to say is 3GB is perfectly adequate for that CPU, you'd have trouble running anything that'd need more than 3GB. Even today most games won't even need 4GB. TF2's from 2007 aswell and won't even use 1GB, rather around 600MB.
If you actually need more RAM then the next thing is to find what you currently have.
You'll also want to know what motherboard you got.
You can use CPU-Z for both.
http://www.cpuid.com/downloads/cpu-z/cpu-z_1.72-en.exe
Just download it, put it on a USB-Stick and the install it on that pc.
You want the motherboard and memory tabs. Check in the SPD tab how many slots you have and how many are in use.
And please take screenshots, don't take photos of the screen.
The problem is that the Q6600 can use either DDR2 or DDR3. It depends on what your motherboard is capable of. DDR2 is obsolete and hard to find these days. You can't mix them, so if you mobo can handle DDR3 it'd be best to replace it. Same if all slots are full, you'd have to replace the memory since you can't add any more.