The P280 is really nothing special, compared to the majority of the cases on the market - it does away with the divided zones of the P183, does away with support for the CP PSUs, does away with the more solid, removable drive cages and does away with the multi-ply construction of the side panels. In return it's lighter and has better ventilation. Check http://www.silentpcreview.com/article1229-page1.html
P193 versus P183 - the P193 has a larger side fan and an extra top 120mm fan. The P193 also supports EATX motherboards so is a bit longer and heavier. My P183 had an eSATA port on the front too.
If you're planning on buying the "normal" short-PCB GTX670s, they would fit fine in your P150, or indeed in any ATX case, as the cards are only 9.5 inches long, so fit inside the footprint of an ATX motherboard. I'd be a little concerned about ventilation issues using a pair of them in a P150 though - you probably want a case with a side intake of some sort to make sure the graphics cards aren't starved for air, although this of course will allow noise to escape more easily.