As to the PSU...
I believe it is this unit: http://www.lc-power.com/en/product/netzteile/standard/lc600h-12-v231/
I can't vouch for its quality, but it has +12V specs that would put it in the 500W category. So treat it as such. The +12V rail is the one that does all the heavy work in the system. It powers the CPU, GPU, fans, drives, etc. That is the important rail to be aware of when looking for a PSU. It would be adequate for the GTX 660, but a bit marginal for the R9-270X.
http://www.realhardtechx.com/index_archivos/Page362.htm
Yes, there were some 775 boards made with DDR3 memory systems. That was a time when the transition from DDR2 to DDR3 was taking place. Here in the US, new 775 boards with DDR3 are pretty scarce. They have been obsolete for years. The only high-end boards that would be suitable for the Q9550 would have to be found used. I would suspect the same is true in your country. Asus, Gigabyte, Asrock, MSI, are some of the better names. But me picking one out for you here and expecting it to be available in your country would be hard to do.
Your board is still adequate for satisfactory gaming with the Q9550. And the Q9550 is still a capable quad core for gaming with a good card. The GTX 660 would be a good card to use with it. As would the R9-270X. But anything faster would be bottlenecked by the CPU.
But I'm still confused by your statement on the memory: "I looked and looked for 2x4GB DDR2". That would be 8GB of RAM, which you couldn't use. The most you can use is 2 x 2GB. And if you have 32 bit Windows, that is all you can use anyway.