Gaming PC in HTPC casing (around 500$ - 800$)

Well, that video card is a bit high. At that price I would get an HD4870 with gddr5 memory. Can you use newegg for your purchases?

Apevia makes low end equipment, I have found their fans to be noisy and cases to be flimsy. I would expect their power supplies to be about the same quality.

For gaming your really want the best PS you can afford to power your video card. That being said, you could pick up an Antec earthwatts 500W for around $60. It may cost a bit more up front, but then once you found out the hard way that the oem PSU is a piece of crap, you would end up spending more anyway.

If it was an HTPC, I would say ok, maybe you can get away with an inferior PSU.

HTPC's are a bit strange, they don't come equipped with silent fans. If you want silent fans you really need to use 120mm fans at slower rotational speeds. I don't see most HT boxes with 120 fans, only at the upper end with antec or silverstone.
If you can find a mid priced HT box with 120 fans, I say go for it.

Honestly that's waaaaaaaaay too much motherboard for a lowly 7850 CPU. If you want a full sized ATX board then go with 770 based board (I recommend the Gigabyte Ultra Durable). If you want a board with a single PCI-E slot then I would go with

ASRock A780GMH/128M $69.99
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

or

GIGABYTE GA-MA770-UD3 $74.99
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

and put the savings toward a more power CPU. After all the 7850 runs pretty hot so it will be hard to get it cool & quiet especially if you want to overclock it. If you think you may go crossfire in the future then I also think you should consider this board.

GIGABYTE GA-MA790X-UD4P $109.99
http://www.newegg.com/Product/Product.aspx?Item=N82E168...

As for your case it doesn't have the greatest PSU, but it should be o.k with a 4850 and an Athlon X2 or Athlon II X2. I also recommend you get some extra fans as those two Apevia fans won't move alot of air. Replace them with some xigmatek fans, or some Blue LED Antec fans if you want the blue LEDs, that push more air.

Hi, thanks for your quick answers,

Which CPU do you suggest to go this my current MB ?

CPU AMD ATHLON II AM3 250 X2 3.0G 98$
or
CPU AMD PHENOM 9650 Q-CORE 2.3G 140$

Ok, my PSU is probably cheap but 500 W of power is it enough ?

Thanks,