Alright, I could give you a long answer or a short one. :P
The first computer you mentioned would indeed cost more than 500. If someone would be attempting to sell it for 500 it's fishy. Aside from that the powersupply is actually too weak for the rest of the specs, causing it to break rather quickly or not run at all.
I personally use a quad core with multi threading so it uses it as 8 cores instead. You really won't need anything more than a regular quad core for gaming. Anything more will be usable once you are going to video-edit, are running an audio studio on your computer or have other visual or audio work of that sort.
The graphicscard on the first noted setup is rather weak and won't last for years if you want to play the most recent and upcoming games. However the line of "** Upgrade to Higher Video Card for $125 to play all the newer games on high settings **" scares me. That's an obvious sentence to trick the people that don't know much about hardware. Yes, a 125 dollar graphics card can give you enough to play all the newer games, however it does really depend on what graphic card it would be. And there's a chance you could get a cheaper one with the same result :).
I would however higher the amount of RAM you have selected in the setup as 4gb is the current minimum.
Last but not least : Gigabyte AM3+ Motherboard <- This does not give enough information. There's a TON of gigabyte am3+ motherboards. You'll need a version or serial number so you can look it up online. And make sure you have at least 2 PCI-Express slots and multiple slots for your RAM memory. Also check the compatibility of the motherboard with different CPU's. This way you'll know if you can use the motherboard long-term or if it is not possible to get more out of it than you currently are buying.