Ohms are quite simple to understand really. You get an amp that will take a 4Ω load, you can run either 1 4Ω cab or 2 8Ω cabs & it'll put the full 500 watts to it or 1 8Ω cab will take about 350 watts.
When you start asking how loud a thing will go, things start to get a little more complex as watts have little bearing on volume.
Couldn't tell you that much about GK stuff, but recently some folk was saying that the stuff with gold speakers is rubbish in comparison to the other GK stuff.
There's some good deals going on TC Electronics stuff just now like this...
http://www.dv247.com/guitars/tc-electronic-bh500-bass-amp-head-and-bc212-full-stack--99777
Depends what your budget is though.