How much CFM do I need for this system?

No overclocking/crossfire/SLI.

I need to buy a front case fan from one of the following. Right now, I'm under the belief 1 front and 1 rear fan is enough for my system. Please let me know if I'm mistaken. Here are the fans:

Arctic Cooling 120MM 400-1500RPM 56.3CFM 10-23DBA Fluid Dynamic Bearing - $5
Scythe Slip Stream Kaze Jyuni 120MM Fan 800RPM 40.1CFM 10.7DBA Sleeve Bearing - $6
Coolermaster R4-C2R-20AC-GP 120MM Long Life Sleeve Bearing No LED 2000RPM Black Cooling Fan - $6

This is my system:
CPU: Intel Celeron G530 Sandy Bridge 2.4GHz LGA 1155 65W Dual-Core Desktop Processor Intel HD Graphics BX80623G530
CPU Fan: fan that comes w/ CPU
Motherboard: MSI H61M-P21 (B3) LGA 1155 Intel H61 Micro ATX Intel Motherboard
Video:ASUS EAH6670/DIS/1GD5 Radeon HD 6670 GDDR5 1 GB Video Card.
RAM: G.SKILL NS 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1333 (PC3 10666) Desktop Memory Model F3-10666CL9D-4GBNS
Harddrive: Hitachi Deskstar 7K1000.C 500GB SATA2 Hard Drive 7200RPM 16MB Cache 3.5IN OEM
Case: Coolermaster Elite 370 Black Mid Tower ATX Case 3X5.25 1X3.5 5X3.5INT No PS
Power: OCZ ZS Series 550W 80PLUS Bronze High Performance Power Supply compatible with Intel Sandy Bridge Core i3 i5 i7 and AMD Phenom
Optical: Liteon IHAS124 24X DVD Writer SATA Black OEM

I'm not sure what the RPM and CFM of the rear fan that comes with the case is... but is it possible to tell from my build whether the Scythe fan is sufficient? I basically want the most quiet fan, but still want to have enough cooling for my system. The computer will be in sleep mode about 17 hours/day. It will be used about 7 hours/day: 4 hours surfing internet/watching anime, 3 hours playing Diablo 3 on medium settings, which has these system requirements:

Recommended System Specifications
OS: Windows Vista/Windows 7 (Latest Service Packs)
Processor: Intel Core 2 Duo 2.4 GHz or AMD AthlonTM 64 X2 5600+ 2.8 GHz
Memory: 2 GB RAM
Video: NVIDIA GeForce 260 or ATI Radeon HD 4870 or better

I would suggest either a ball bearing or fluid dynamic bearing fan as they are quieter. I've used sleeve bearing fans - CM Sickle Flow (the R4's you're looking at) - and they are mostly really great as far as CFM and noise goes, but I often hear a "tick" sound from the bearing. This happened on 2 of the fans. I still use them, it's not that bad now that the PC is on the floor, but when it was on the desk I could hear it.

FDB fans won't have that issue.

Other than that, most fans under 1400rpm will be nice and quiet. CFM won't matter too much. Scythe S Flex FDB fans are great. So are Gentle Typhoons.

The Arctic Cooling fan you listed looks fine but only if it has speed adjustment built in, or maybe you have a mobo fan pin you could use.

800rpm is probably a little less than you want, although it will work (that Scythe fan)

Yes, it should be fine with just a rear exhaust although hopefully it is placed near the top and not the bottom (or if at the bottom, the PSU is at the top which also acts as an exhaust).

Keep in mind with a negative pressure case (more exhaust flow than intake) you tend to build up more dust, so you'll want to open it up and clean out the case and heat sinks every 4-6 months.

interesting, according to their website it supports 120mm fans at the front too:

http://www.coolermaster.com/news.p [...] ws&id=6200

is this an error by them?