Sorry guys, but there is so much BS going on in this thread, it's painful to see as a hardware enthusiast.
Comparing GHz-s and cores is like if you would try to compare megapixels for cameras: It really doesn't work that way.
Yes, sure, a stronger CPU will have 4 or more cores (preferably real cores and not HT cores) and a relatively high (3+GHz) frequency but you still can't compare CPUs just by that.
An AMD FX-4300 has 4 cores and runs on 3.8GHz, will it be faster than an Intel Core i5 3570K which also has 4 cores but only runs at 3.4GHz? No. In fact, the FX-4300 is about 1.5 times as slow as the 3570K. What you should use for comparing CPUs though is benchmark scores. A good site that I can recommend is CPUBoss
for CPU comparison.
When it comes to RAM, nowadays you almost always want to have 4GB or more because your operating system alone can easily eat up 1GB+ RAM, and additional applications (like having an IDE, Steam, Skype, torrent and a web browser open) 2-3GBs. Personally I find 8GB to be the sweet spot right now (16GB is overkill unless you're doing a lot of rendering and 3D modelling), but if that's too pricey for you 4GB should be fine. Still, avoid 2GB or lower at all cost. Also make sure that the RAM is at least 1300MHz and has a CL9 timing.
If you want to use the rig mainly for gaming then the main component that you should worry about is your GPU. Fetch as much money into it as you can and shop around. AMD GPUs tend to provide a bit better price/performance ratio than Nvidia's cards, but I personally tend to prefer Nvidia for their rock solid drivers, CUDA and shadowplay.
When it comes to the power supply you should buy something from the better brands (Corsair, EVGA, Cooler Master, Seasonic, FSP and a few other) with at least bronze efficiency.
There isn't much to be said about motherboards, my recommendations depend on the socket that you would go with (LGA-1150/1155, AM3+ or something else), but usually the good brands are ASUS, ASRock, MSI, Gigabyte (probably in that order).
There are a few other components that I didn't talk about now (case, HDD, SSD, optical drives, chasis fans, CPU coolers, etc.) but if you have any questions please go ahead.
Also, as a general rule of thumb you should never buy a prebuilt computer if you have at least the smallest idea on how to build a PC. Buying components one-by-one means way better price (you can usually spare around 15-25% just by doing this), longer warranty (there are components out there with 5 or even 10 years of warranty, but with prebuilts you usually don't get more than 3 years), and a few other extra benefits like if your graphics cards goes wrong you can just send it back and use your integrated graphics in the meantime, while with a prebuilt you have to send back the whole thing, etc.
As I said earlier any of you guys need help with hardware or want me to build a best bang for the buck PC for his needs feel free to send me a PM or ask me here.