3.6 GHz 4-core, or 3.1 GHz 8-core?

yes.
the 81xx is a better chip than the 4100

What exact processors are you looking at and what are you doing with this computer. Cores and GHz really don't matter much anymore and really isn't a good way to tell how good a proceesor is. AMD has fallen behind in performance and when compared to Intel usually the Intel CPU will win out even if it has less cores and "slower" clock speed. Intel microarchitecture is faster and more effcient than AMD so that lets it do more work at lower clock speeds.

its going to be a gaming rig but it needs to be able to handle Battlefield, Call of Duty and such with no lag and the processor im looking at is a AMD FX-8120

Quad-core. No game currently uses more than 4 threads, and I highly doubt developers will use more than 4 threads in their games for at least 2 more years, if not even longer than that. Running an 8-core for gaming is basically only using the half the processor. However, if you want to also record your gaming with Fraps or something similar, having 6 or 8 cores is useful, because Fraps can utilize those inactive cores without taking any cores away from the game. You'd likely notice little to no drop in FPS when recording with Fraps with more than 4 cores.

But that ability to record while playing with no decrease in performance has to be worth the extra $100+ to you. Personally I'd suggest the best-available quad-core on the market.

Well you hit the nail on the head when you mentioned recording gameplay because i need this rig to kick off a youtube channel ive been planning for the past couple months and with the special offer going on right now from iBuyPower when you build a computer i can get the AMD FX-8120 for only an extra $35

Well then I think you have your CPU =) I might add that rendering a finished video will also be noticeably faster with 8 cores compared to 4.

One last warning: Performance per core will be slightly slower with the 8 core at 3.1Ghz. Since your game will only be using 4 cores regardless, and all FX CPUs use the same architecture, 4 cores running at 3.1 is a little slower than 4 cores at 3.6Ghz. But you can always overclock the FX8120 to 3.6, should be easy =)

Awesome =D now just one more question, should I get a better graphics card than the NVIDIA GeForce GTX 650 1GB?

What games are you playing, what resolution are you playing at and what graphic settings do you want to play at? The GTX 650 is a low end entry level gaming card and you won't be able to get good FPS at max settings on high resolutions.

absolutely. Unfortunately, that graphics card will easily bottleneck your system (it's too weak to get the maximum performance out of your CPU). I don't know what your budget is but usually when choosing a CPU-GPU pair, the GPU costs more than the CPU. I'd look at the GTX 660 superclock, 660 Ti, or even the 670

im on a budget unfortunately the best id be able to get is the 650 Ti Superclocked would that suffice?

What resolution are you playing at and at what graphic settings.

im most likely going to be playing 1080p mid graphics

You'd be okay with CoD, but you'd have to run a game like BF3 on minimum settings, and your frame rate wouldn't be very high. Hate to be the bearer of bad news, but you might want to look at reducing the cost of a different part of your build so you have more flexibility with your graphics card. See if you can rework the build a little bit to afford a 660

It's up to you. If that is all you can afford than I guess it doesn't matter very much since you can't get anything better but I personally would save up and get a better video card. The 650/650Ti really isn't that great for gaming.

for reference, here's some benchmarks for the 650 Ti, specifically BF3 at medium settings (there are other games in the article too). It should help you decide if it's good enough for you.

http://www.tomshardware.com/reviews/geforce-gtx-650-ti-...

Yeah i'm going to have to go with the 650 Ti Super for now but i will save up for a 660 or better