The benefit of dual channel is that you can essentially move twice as much data as single channel.
From your website, there is a Seasonic 520w psu that is really very good: http://dynaquestpc.com/index.php/components-2/power-sup... Sounds like it is overkill for your system, but it would allow you to upgrade in the future if you want to. That psu would actually power a single video card, near top of the line system right now.
are you sure it's good? im planning that in future im upgrading my cpu to an i5 quad core and an ATI Radeon 7950 DDR5. is that psu stable for my current system or it will last for a couple of years??
The GPU is normally the single largest draw of voltage in a performance-oriented system, and if you are planning on a 7950, the PSU should be one you trust to really be up for the task. Seasonic is a great (arguably the best) PSU manufacturer on the market, and the M12 or S12 520w units are both excellent (only difference is the M is modular). It would most likely be enough to handle your future plans. However, I would seriously consider going up to the 620w Seasonic M12 or S12 models. That nets you a 12v rating increase of 8 amps, up to 48 total- which is where you really need to make sure you have enough power.
FYI, I have run a HD6850 crossfire with an overclocked Phenom X4 965 on the M12II 620w unit and wasn't breaking a sweat. That's in theory 200+ watts CPU draw and 250+ watts GPU draw- far less than your proposed future upgrades.
No, your current system will only draw as much power as it needs. The psu will not *force* extra wattage to it. The 620 would be a better idea, it was just outside of your budget. If you want to understand the quality of psu's a little more, here: http://www.eggxpert.com/forums/thread/323050.aspx
I currently have a tier 2 psu, my wife has a tier 2 psu that has been going for almost 5 years, 24/7. Those seasonic's are also tier 2. When you choose to go with a cheap psu, it might work for a while. The issue is, when it finally dies on you, it can literally destroy your system with it. Cheap = not quality, not necessarily having to do with price.