I have been researching Alienware since christmas when a friend purchased the 17" model.
1. basically everything is upgradeable with the Haswell architecture. You can upgrade the processor, memory up to 32gb, Hard drives and SSD. Graphics cards between the Nvidia 765, 770 and 780. The comparison i've read is the the SLI 765 will get about the same result as a single 780. The SLI 780 will get double the performance of the SLI 765. If you configure at the Alienware site the 18.4" base model starts at 2049. The model is 8 gb ram 765 SLI and a 750 gb HD and only choice is Windows 7 home premium. The next model is more upgradeable and starts at 2099.
The second model is basically the same as the first but comes with a 500gb HD 8gb hybrid and 64 mb cache but is upgradeable for 100 more to a 1 TB HD with a 80 GB MSATA SSD. The 80 gb is for caching only and not boot. For 300 you can opt to 256 GB MSATA boot drive with a 1 TB HD. A note about the drives you can max out the performance by taking out the optical drive and add 4 256 GB SSD's RAID 0 (not available on the dell site but can be done). Also the second option you get the choice of Windows 7 home or premium or windows 8.1 home or premium. You can also configure to 16gb of ram (max is still 32gb) .
The 3rd configureable starts at 2999. This model starts with the i7-4800MQ and the previous 2 options used the 4700MQ. This model also starts with the Nvidia 770 and can be configured with the 780 for 550 more. You can also go from 16 to 32 gb ram for 150.
All 3 come with the Broadcom new wireless standard 802.11 AC protocol.
These are the option on the Dell site. There are many more options on Ebay with many more mixing and matching of components. You can find many more mixed and match configurations on ebay but most are refurbs.
Maxed out the Alienware with the most powerfull processor, Nvidia 780, 4 256 SSD RAID 0, 32 gb ram can run over 5000.
To me for my use the 780 isn't needed and the 765 is actually overkill but you can turn the discrete card off when running on battery to conserve battery life.
I like lots of hard drive space so i'm looking at the 2nd model with the 256 GB boot drive and 1 1TB HD. I have options I could replace the 256 GB later with a (480 gb, 750 gb or 1 TB when proces drop even further). The system comes with 2 hd/ssd slots and a MSATA (mini SATA) slot. The MSATA is usually used for cache drives but you can use it as a boot drive. I have configured it with Windows 8.1 for around 2500.
If your interested look on ebay and see all the options and the variations in price. They have systems with 32 gb of ram for less then 2500. On ebay it does state these system are refurbed by Dell and the warranty is honored from original purchase date. Most systems are a month or less old so you get almost the full factory warranty. I did see some comments where someone purchased one and it did have scratches.