i want to build my system for mid range games, web & graphic designing, and multimedia purpose
my budget is 600-620 $ with graphic card
Processor : Intel Core i3 2100
Mother Board : for ATI HD 6790 sapphire with 4 DIMM ( brand : Intel or Asus )
Graphic card: ATI HD 6790
Memory : 3x2GB
Power Supply: which suite for graphic card and mother board not more then 45$
Pc Case: not more then 35$
HDD : 500GB SATA
For your ram you will want 4x2 other wise your motherboard will not run it in the correct channel as 3x2gb are for triple channel board setups. For your board an asrock z68 extreme 3 gen 3 would be a good choice, and for the case a bitfenix outlaw would be a good choice.
We're at $632 and if you want to cut it down by $20 or so then you can go with a micro-ATX board. I didn't shop for combos so you can probably do a little better price-wise. You can go with a cheaper chipset since you're not overclocking (because of the non-unlocked chip) - I wouldn't argue with an H67 board. Don't go with a PSU with less power than the CX500. The OCZ that popatim suggested is good but you can't really bank on rebates.