Is it worthy to set a 8GB DDR2 ram(2GBx4) with a core 2 quad q6600 cpu? and what is the best budget GPU for such combination... now I'm using a nVidea GeForce GTS250 512MB. I'm thinking about a GTX560TI 1GB OR GTX 660 2GB... What about it with my CPU and RAM memory

i have 4GB (2GBx2) RAM and i'm happy about it .

System requirements for games may vary and there's a standard preset system requirement that may be well balanced for most games.

Source engine games are not very GPU intensive but rather CPU intensive. The source engine uses a lot of rendering / post-rendering power which is one of the most intensive functions of your CPU, your GPU is important but most of the back end stuff is done by the CPU. 

There are some games that run on most potato PC's - the Quake games were a notable example.

In general, our target is 16GB of RAM for gaming computers in the current market. You want an SSD of some sort, generally 240GB or more, and it's generally worth while to supplement that SSD with a spinning drive that gives you at least 1TB of extra space. Remember, SSDs don't really offer much performance to games outside of asset load times and games are getting up to 50GB installs these days so putting all your games on a SSD gets cost prohibitive quickly and delivers little benefit.

For a Core 2 quad q6600 CPU, the GTX 660 Ti seems much viable.

I'm currently using a general potato PC with just 2 GB of RAM.... -_- I miss my old PC.

I barely game as much as I used to but soon enough I'll return to play some old games and even given Quake Champions a try, though, I don't like what they did to it (Quake).