I have no idea where you get your information form, but the cpu is not that big of a deal.
The FX-4100 is a decent choice for the price if he was absolutely budget constrained. A i3-2120 is better for ~$20 more, but considering he's putting a card from 2008 in his rig, it probably came down to a budget concern.
The CPU itself will probably give around 5% lower FPS average in most games (with Starcraft 2 in high supply high demans situations being the outlier that's bound by CPU performance with a 7850 level card) at 1920x1080 high settings compared to a stock i3 2120 with a 7850 is not a super high end card. With a 4870, the difference would be minimal since the 4870 will be the bottleneck in any modern game. A good cpu is awesome to have, but given price constraints the FX-4100 at 4.2ghz is not too bad. The FX-4170 (A FX-4100 at 4.2ghz stock clocks) does alright in game benchmarks at real playable resolutions.
Source for a stock FX-4100 v.s. i3 2100
For everyday work that doesn't involve encoding, the difference will be negligible. Considering that even 1080p flash videos run fine on ULV i3s found in cheap laptops, you'll be alright.
A real point of concern is in your case and psu. The Storm Scout has a certain style if you like that, but cooling performance is not very good. That's not a show stopper if you're okay with that though.
The big problem is your PSU. It looks like you got a generic 850w, which if correctly rated would be massive overkill for your system, problem is that with generics, they're usually untrustworthy once you exceed half the wattage rating and some of them will burn out, taking other computer components with it.
Even if it doesn't die, voltage and ripple can be way outside of ATX spec and that could take components out also. The power supply is the one place you never want to cheap out on and frankly a $40, 850w PSU is not something I would trust running my pc. I've seen some PSUs that even during idle pulling only 20% of max load putting out ripple 3x beyond ATX spec, which will lead to a slow but sure death for your system.
I would switch out the PSU asap for something reliable. If money is a issue, I'd recommend a
Corsair CX430. Never, ever, ever buy a generic PSU unless you can find a good professional review that opens it up and looks at the interals as well as does benchmark checks. Anandtech and Johnnyguru do great psu reviews.
There are only a few PSU companies that are truly trustworthy without having to check for model reviews:
Tier 1
Corsair
Seasonic
PC Power & Cooling
Antec
Enermax
Silverstone
Tier 2
Sparkle
FSP
A generic PSU from china with no UL number that I can find from a manufacturer without a website selling a 4 lb including packaging (weight usually correlates with PSU quality) $40 850w psu raises way too many red flags and I would not trust it to keep my PC alive if I were you.[/QUO
i got the HD 4870 for free and ik that it isn't gonna do very well in modern games wasn't really expecting it to lol.. and from some good benchmarks the FX 4170 does do about 5% better than the i3 2100.... and the psu is ok for now until i get a new gpu it has 2 12v rails rated 27a and 28a