If you have a quality PSU it is probably fine. I think your main problem is the WD Green. Greens are great storage drives but they are slow........too slow for a C: drive in my opinion. Eveyone I have talked to regretted getting one as their boot drive. The 5400RPM spindle speed just can not keep up with a 7200RPM drive.
Never heard of it. I can not really find anything in English about it either. Generally the quality brands are known internationally. Corsair, Seasonic, PC Power and Cooling, Antec and Silverstone are all top of the line quality brands.
I still think the 5400RPM Green drive is your problem. It will be slow to find things because of the slow spindle speed causing pauses. As I said earlier everyone I have talked to has regretted getting one as a C: drive. They are great for storage though.
The computer does not crash right? Just hangs or slows down when you try to access something on the hard drive?
You only need a 400w PSU for that system. If you plan on upgrading your graphics and doing some extreme overclocking then a 500-550w unit from a quality manufacturer is fine for any single graphics card. You really only need that ( excellent ) 650w Corsair if you plan on running 2 high end graphics cards in Sli / Crossfire like 2 x 5870s or 2 x GTX 470s. You really can save some money there. Stick with Corsair, Seasonic, Antec, PC Power and Cooling or Silverstone and you will be fine.
Not sure if you are in the US but newegg has the Corsair 400CX for $29 after rebate. It is PLENTY for your computer and will even run a HD 5770 and maybe even an HD 5850 just fine. It is really a 500w unit.