It's a lot better, it would eat what you currently have.
I've honestly never even heard of a Radeon HD 7450 up til now, it's probably on the lowest end. The 7950 is like 10x better.
The FX-6120 is from the Bulldozer line, which is known to be pretty bad(being nice) at games.
10gb of RAM imo is all it has going for, but you won't need that. I have 8gb of RAM on my system, and I barely go pass 4gb even with 2-3 different games running at the same time.
His build has an SSD which is where I install OS and it generally speeds up the whole system.
As for Intel vs AMD, the former has a better architecture for single-threaded programs which is mostly what games are right now. The FX-6300 is a great improvement over its Bulldozer sister, but it's mostly for multi-threaded stuff such as video rendering.
That said, it's not bad for a gaming CPU if you want an AMD build. You can opt to get the FX-8320 if you want 4 modules/8 cores as opposed to the 6300's 3 modules/6 cores.