My First PC custom Build?

I just read and understand how to build a PC. and after finding all the part i need i wonder if i choose the right one. Remember that this is a BUDGET build. _CPU=AMD Athlon II X4 620 2.6GHz (it is a quad core so i decide to choose it) _Motherboard=Asus M4A785-M (socket AM3 so it should go with the... show more I just read and understand how to build a PC. and after finding all the part i need i wonder if i choose
the right one. Remember that this is a BUDGET build.
_CPU=AMD Athlon II X4 620 2.6GHz (it is a quad core so i decide to choose it)
_Motherboard=Asus M4A785-M (socket AM3 so it should go with the cpu)
_Video Card=Sapphire ATI Radeon HD4650 512MB DDR3(Why did the put DDR3?what does it mean?)
_RAM=NEED Help.the motherboard support DDR2 so do i choose DDR2 or 3 and what about the PIN COUNT? and this is what the motherboard spec say "DDR2 1200(O.C.)/1066/800/667"
_HDD=Seagate Barracuda 320GB 7200rpm SATA 3.0GB/s
_Monitor=Hanns·G HH-181APB Black 18.5
_Case=APEX TX-381-C Black Steel Micro ATX Tower Computer Case(not a very good one but it is acceptable)
_Power Supply=MY MOST CONCENRED Thermaltake TR2 W0070RUC 430W(i am thinking of going with this because the Video Card said 400W power supply recommend ) and i know it is cheap and not execllent but i already spend enough on other thing.
_DVD burner=can choose later.
Update: So which one is better Athlon2 X3 2.9GHz or Athlon2 X4 2.6GHz?
some people said the quad core is better then 3 core and it is also more expensive.
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