I won't give you parts and such - but I will give you my 2 cents of advice.
For $900 you can definately buy something that will function and likely function well. You should plan for the future needs - as such a solid quality PSU manufactured by FSP, SeaSonic or Zippy would be highly reccomended.
Your Case selection should have sufficient airflow - with a minimum of 2-120mm fans in and 2-120mm fans out. The airflow should move across the motherboard with the top/rear fans blowing out (heat rises.)
Consider even an inexpensive aftermarket CPU Cooler - nearly all stock coolers are 'good' but not great. This is something you can do later - but if it fits into your budget - do it.
Buy a quality MB - Asus is my favorite brand, as I have had good luck with them - ASROCK and GIGABYTE have their fan bases as well. DO NOT buy a crap no-name MB - you will be sorry.
You can go cheap on the CPU - if you have to. Games take more GPU power, but I would still consider a dual or quad core. Intel I5 is about as high as you need to go for a CPU - as the I7 is a bit too much CPU for gaming.
For now - if you can only afford 4GB of memory get a single stick. make sure it is compatible and from a solid brand - so you can get a second in the near future if needed.
Get the best GPU you can afford.
Good Luck.