RAM that is rated to DDR3 2000 as this is what the M'Board was designed to work with. The fact that it's also the most expensive stuff doesn't help but it will be as fast as the M'Board can use and it will work with all the Memory Slots populated.
Personally though I prefer Gigabyte M'Boards but may be that's just me, I've had excellent results with Gigabyte M'Boards in the past and less reliability with any other brand.
http://www.giga-byte.com/Products/Motherboard/Products_Overview.aspx?ProductID=2986&ProductName=GA-EX58-UD4P
If nothing else Gigabyte has better Support Lists for CPU's and RAM.
From years of experience I prefer Antec Power Supplies as they just offer better protection to the Insides of the computer case
http://www.antec.com/Believe_it/product.php?Family=MTQ=
My preference for Video Cards is ATI because they have not had the issues of outright failures that NVidia have and there have been no statements of Design Flaws leveled against ATI like there have against NVidia but that's just me however if you do chose a NVidia Chip Set make sure to run it cool and not push it hard at all and never overclock it.
As for RAM I like Corsair Band RAM
http://www.corsair.com/products/dominatorgt/default.aspx
Now for your OS depending on where you have ordered it it could be anything or it may be both. Some are supposed to be shipping the 32 & 64 Bit Install Disc's of 7 together while other places are specifying which they are selling.
As a general Rule of Thumb if they do not list anything it is a 32 Bit Version and can not utilize more than 3.2 GIG or Total System RAM. That includes the Video RAM as well as RAM fitted tot he M'Board.
It's a waste of money fitting more than 3 GIG of Triple Chanel RAM to the M'Board in a system like this so you really need to know what you have ordered.
I don't have a clue what Apple ships it's Snow Leopard as and their Web Site has no information about it and even less in the way of Technical Specifications on what it comes on.
I hope that is of some assistance to you.
Col