Anyone have a 2013 iMac with 4GB video card

I'm trying to make a purchase decision between a maxed out 27" iMac or a low end 21.5" iMac+Gaming PC. Does anyone have a 2013 iMac with 4GB Video Card who can tell me how games perform on it? I will be planning to use Steam through Boot Camp. I play a wide variety of Games. Old stuff, new stuff. Currently playing Brothers on my 2009 iMac with 512MB video card and it stutters on the cut scenes.

I saw that there are stats on some benchmark sites for the NVIDIA GeForce GTX 780M 4GB GDDR5, but I'm not reality sure what they translate to in real life use.

Any insight is appreciated.

Thank you,