Well the cooling fan is necessary but liquid cooling is not. You could prolly save some money by going with an air cooler.
Kind of the same? I'm not sure what you mean exactly, but I think that an Athlon II X3 CPU would perform similarly in games. The quad would be better for multitasking, but it would not be much different than the X3.
The main hard drive will carry the operating system. Some folks like a second hard drive for Data storage, in this case the data drive is smaller than the boot drive, which doesn't make a lot of sense. skip the data drive.
The video card 1,2 and 3 means the system is capable of using multiple video cards. For the 870 chipset, I would NOT recommend 3 video cards, 2 at most.
The Hard drive and data hard drive are the same type of drive, its just they're referring to the data hard drive as extra storage, which would be used more if you had an SSD as the "Hard Drive" option. As is you could do with just the 1TB Hard Drive and negate the Data hard drive.
The Video Card 2 and Vid Card 3 would be if you were running multiple Video Cards in one system, commonly referred to as SLI or Crossfire.
If you're talking about the hard drive cooling fan then no, it's not a must-have.
As far as making it more cost efficient, that would depend on how much you were looking to spend.
Which would cover you on the wireless side, however you can also buy USB 802.11B/G/N for around the same price as that adapter card. Wireless N is the newest spec and is faster then B and G. That being said you have to have a Wireless N router to use it so that may also be a factor. However I'd say just buy a USB Wireless N adapter, it will work on B and G, and you have the upgrade path to N if you don't have it already.
Wait, does CPU really matter while running a high-quality game?
If so, is there a big difference between AMD Athlon™II X4 640 Quad-Core CPU w/ HyperTransport Technology and AMD Athlon™II X2 250 Dual-Core CPU w/ HyperTransport Technology ? I dont want to waste 50 bucks