Macbook Pro i7 w/ 8GB of RAM   I am up to 3 VMs running at once as well as host based activity (web browsing and such).  Though a Dell Alienware should work out the same. 

Well, i've found nothing better thant the Asus ROG :
Intel Core i7 Sandy Bridge
16 Go DDR3
1,5 To split in 2 hard drive

If you are new to this field start with minimum necessary.

I would recommend to buy a lighter laptop (an used Lenovo or Dell with an Intel i5 will be perfect), with a big screen. Also buy one or two used servers that will host your targets. It will cost less and you will benefit more (you can even reuse the hardware later).

I recommend you a second monitor. I am using a desktop with dual monitors. Sometimes I use my laptop connected to a 24” 1920x1200 monitor. It is very good for the eyes, and you have a lot of workspace. I read somewhere that working on only a monitor is like eating on the table from the aeroplane, and I agree with this.

If you still want to go w only a laptop, a SSD is a must. You can have a Dell XPS, 17" 1080p, SSD, 8GB RAM for about 1500$.

If you have more money... invest them

I'm no professional, but I have $400 ThinkPad X120e that runs like a champ. I got the model with the dual core processor. Upgraded the RAM and put in an SSD, it works just fine for everything I want it to do. I can run virtual machines, LiveCD's, etc., without any problem. I imagine I'd want something more powerful if pentesting was my fulltime job, but it should be fine for a hobbyist or just wanting to learn some new skills.

I guess it depends on what you can afford and what you going to attack. I know people who use a cheap 300 pounds toshbia laptop with ubuntu on it.

I know some people also who use a really cheap old laptop as only testing they do is web apps and all they really run is burpsuit.

I use for work a HP 8840 I think it is. And I run ubuntu on it at the moment I also have an encrypted hard drive to protect any customer data.

I agree with all the points listed.

In essence get as much memory and CPU as you can afford as this will give you the flexibility required to run multiple VM's. Multiple disks are a real necessity if you do not want performance issues. I run my VM's from a USB drive and find the performance satisfactory so try that if you cannot get something with multiple drives.

I would not buy cheap to upgrade later as that is a waste of resources. Get the best you can for your budget as you can always use it for something else down the line or sell it to another enthusiast if you decide not to pursuit this.