Personally I am a fan of a solid workstation as the primary computer, and then a light weight laptop or possibly a tablet for your mobile unit. (I currently use an older i5 based desktop system I assembled myself, and an ASUS transformer TF301 with keyboard dock for my mobile work. Tablet is for content access and review, not editing, but I haven't really done any tethered shooting with it. Might do that tomorrow as a trial maybe.)
If you are technical minded, I strongly encourage assembly of your own system. (And for anyone I discourage Alienware products. You pay an arm and a leg for an ugly box around a computer better designed to fry eggs on than function as a portable computing device in most cases.)
Step 1 in building your own computer: Pick a quality case. I use an older Antec P-180 (Sadly their newer models of it were far lower in quality last I had seen), and I've had it nearly 10 years. It has held three or four 'computers', each with a handful of upgrades. A case can always be reused, and for better or worse ATX is likely here to stay. (Which sucks, because that motherboard layout is less than ideal for heat reasons. Sadly, it is what cases are made in, so mother boards are made in it, and since most mother boards are made to that formfactor, cases get made in it... An annoyingly hard cycle to break.)
Consider what you want, storage wise. I have five or six hard drives in that system, a few of which are there simply for legacy files. You might want to go with a smaller case that holds fewer drives, but I can still add lots more if I really want to. After you have decided how much physical room you need, start considering your processing usage.
For the vast majority of people these days, a mid-upper range Quad Core intel i5, in a lower end motherboard, and 8 gigs of mid speed ram does just fine as your base system. Include a decent quality powersupply based on the parts you use and those that you're considering, (Don't just throw a 1000watt rig in and assume you're doing any good. Many systems will actually be fine with a 300-400 watt system.) and the other odds and ends that you need.
Don't forget your software licenses either. I've known more than a few people who price out a system, order parts, put them together, and then remember they will need an operating system. (And a mouse and keyboard. So many of my friends have ordered new systems and forgotten they need a way to interact with it.)
So, write out how you see yourself using your new computer. What do you really want to do with it? What are your space requirements, (Memory, and physical space?) what might you be interested in using it for? How long are you willing to wait while batches of files get processed?
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