I wouldn't suggest a laptop or ultrabook for gaming unless you go with a company that builds gaming laptops. Ultrabooks lack proper cooling and some don't have dedicated GPUs. You can follow any of the budget build guides on Tek Syndicate if your willing to build a desktop. Here's a few tips for building your own system(laptop tips further down):
Consider AMD CPUs over Intel. The price difference is crazy. Intel motherboards are also significantly more expensive.
Make sure the motherboard has the right socket size for the CPU you select.
You don't need more than 8GB of ram. You'll never use that much. Ever. Keep the sticks to a minimum. Motherboards may have 4+DIMM slots, but that doesn't mean you need to or should fill them all. It's better not to.
AMD GPU's are just as good as Nvidia's. PhysX is supported on AMD GPUs so thats not an issue(as if you want it on. The PhysX water in Borderlands 2 looks like jelly)
Cases are important. Don't pick a case with airflow from front to back. Whoever thought that was a good idea obviously never took a basic physics course in high school. Heat goes up. Fans should push air up. Cable management is equally important. It's quite obvious why. If you can, get a case with filters. Dust is the enemy. The specs on the motherboard you pick will tell you the form factor. Cases do not support every form factor.
If you intend to get your own fans, go with Noctua. They may look terrible but they push the most air with the least amount of noise. Noctua has a guide on their website so you can figure out which fan to get.
PSUs(power supply units) produce very little noise. Don't get a "silent" PSU that lacks a fan. You will regret it. Make sure the PSU at the very least meets the GPU power requirements. These vary from card to card so don't assume they're the same.
Fancy heatsinks are pointless unless you intend on overclocking. Intel and AMD aren't going to sell you a heatsink with their CPU if it doesn't cool worth a damn.
I may add more later.
Gaming laptop purchasing guide:
Ignore anything bad you've ever heard of about Alienware. Haters gonna hate. They aren't retardedly expensive, though the hard drives and RAM are over-priced. Both are easy to replace.
Research research research. Reviews are biased. Find some forums specific to the laptop of interest and do some reading.
For the love of God stay away from the Razer Blade. You can't customize it and it's priced like Apple's laptops.
Mobile processors are NOT the same as desktop processors. Mobile processors can only get so much power from the wall, so don't assume the HD 7970 is the same as the HD 7970m.
Again, I may add some stuff later.