Well it varies, but in general the higher dollar motherboards are more so packed with overclocker and enthusiasts friendly features.
Triple channel in generalized terms means that you have 3 lanes (channels) to run programs, commands, etc through versus the common 2, there is large debate over 2 channel vs. triple channel but in the end you'll be just fine with a dual channel motherboard.
Clock speed is how fast as piece of hardware is running, compare it top a car, if your going 15 mph it'll take you 4 minutes to go a mile, if your going 60 mph it'll take you 1/4 the time to go the same distance. Generally speaking faster is better.
RAM timings are complex but just know that a lower timing leans for better speed.
With RAM first you want to decide how much you need 4GB (2x2GB kits are the norm), most importantly you want dependable RAM from a reputable company (Corsair, OCZ, and G-Skill are all great). As far as performance goes you want the higher speed with the lowest timings at a price that's right for you.
OS does matter, it the interface, it's what makes your computer a computer.
You'll want to go with Windows 7.
As with everything else computer related your options are next to limitless.
You'll want a monitor with a low response time, good colors, a wide contrast ratio, and a decent resolution.
Here are some suggestions:
Newegg.com - Hanns·G HH-231HPB Black 23" 5ms HDMI Widescreen Full HD LCD Monitor 300 cd/m2 DCR 15000:1 (1000:1) Built-in Speakers - LCD Monitors
Newegg.com - SAMSUNG 22" 5ms Height &Pivot Adjustable Stand Black Widescreen LCD Monitor 300 cd/m2 DC 8000:1(1000:1) - LCD Monitors
Newegg.com - ASUS VH232H Glossy Black 23" 5ms Widescreen Full HD 1080p LCD Monitor 300 cd/m2 ASCR 20000:1 w/Speakers & HDMI - LCD Monitors
And if you haven't caught it yet I think everyone here well anonymously agree that NewEgg.com is the best place for computer hardware.