Hello my friend! First of all let me tell you that Gaming on a PC is quite a expensive hobby, you get exactly what you paid for, I'm saying this so you won't be dissapointed later.
PSU stand for Power Supply Unit, and MotherBoard or MOBO is the Main Board where you'll install all the components (RAM, CPU, HDD, etc).
These two are the components you can't skimp on. MOBO will let you upgrade later and PSU is the one will keep you video card juiced.
I'll dare to say that a standart budget for a gaming PC should be 1k, could be less cuz you might find good deals online, but honestly the games you want to play (FPS: Halo, FEAR, COD, etc) are demanding games, and if you want to start from scratch then that means you'll need a Monitor, KB, Mouse, etc.
pretty hard to make it with $500. I'm not saying is impossible but it's hard to build a gaming machine from scratch with that budget. My suggestion is to save a little bit more and maybe with $700-800 you can get a decent gaming system.
About the Parts you posted:
The Freetch Cobra G6100 is a pretty good deal...
http://www.newegg.com/Product/Product.asp?Item=N82E16856121015
even tough it's not compatible with the Video Card or RAM you posted, the video Card is AGP, that's the name of the slot where the card goes and this MOBO has a PCI-E slot, The RAM is DDR2 and this MOBO is only compatible with DDR, I think an option is that you could use this barebone PC with the onboard graphics for a while in the meantime you save money for a good PCI-E card, just add the CPU, RAM and HDD, BTW don't get one stick of 1GB Ram, for DDR RAM get 2x512, let's say, something like this:
http://www.newegg.com/Product/Product.asp?Item=N82E16820148007
That way you'll get the Dual channel working
Also PSU on this case is not a good one, the Amps on the 12v rail is too low, but like I said, you can use the onboard Graphics and then save for a Video Card and PSU in the future. The PSU will be ok for just the MOBO, 1 HDD and 1 Optical Drive.
Ok, I hope I help you a little bit, I dont mean to confuse you with all this stuff, anyway if there is something you want to know just ask, we're here to help
GL :wave: