For this, you can use a free utility like Maricum Reflect that will do all this for you. See the relevant portion in this guide:
http://www.laptopmag.com/articles/upgrade-dell-xps-13-ssd

I upgraded my xps ssd and it worked like a charm. You don't even need to install windows again!

Edit: If you want to keep your old drives, you can straight plug in any new drive. Open Partition utility in windows and just partition the new drive ready to use. (Not even a reboot required)

 

Yeah go for Ryzen 1700 but it's really worth to OC it to ~ 3.9 GHz (people do it even on stock cooling ) also check if your chosen B350 board supports 3200 RAM (last I checked only some X370 boards were able to get RAM over 3000 but that was like 1.5 month ago)

 

If you use Windows 10 you don't have to do anything with your drives just plug them in and boot in your current windows installation - windows 10 in the beginning will manage all components difference so it can take few minutes but you are ready to go with keeping your current programs/games, files and everything.

From what you are planing on doing I think Ryzen will be better - if you decide on i7 7700k you must buy z270 motherboard - you can think about o.c. even in few years but you are ready.

16Gb ram is enough and it's the easiest component to upgrade - stick to something like 2x8gb - and buy some popular brand with popular model - like corsair - you could than find the same ram on ebay or amazon in few years when you want to upgrade.

1070 is fine but wait for amd vega - price may drop or vega will be something huge who knows.
PSU - 600W will be enough to run build like that and even it's future proof as components are less power consuming with each generation.
Case - your choice really - I think you should pick something that you like. Also don't go for mATX if you do not plan to go to lanparties etc. ATX is the most popular for many years and will be for many years still.

 

Since you're basically chaging all your hardware I'd personally go with the fresh install. Also, you don't really have a choice with your OS - new CPU's are no longer officially supported on any Windows other than 10.

As others said, Ryzen is what you want if gaming isn't your main focus (1600 or 1700 if you want to do OC yourself, 1600x or 1700x if you don't). Do be aware that you should read some sort of RAM guide for Ryzen, since it needs some particular configurations to run well (or it did a couple weeks ago at least). I know you don't really want components recs yet, but I thought I should mention that the Asus Prime B350-Plus seems to currently be (one of) the best B350 motherboards. And yeah, you can just plug your old HDDs in and they should work. Installed programs on them however may not.

 

Echoing what others have said about RAM, I'd suggest reading up on how RAM affects Ryzen if you haven’t already done so, but in short:

-Maximum RAM frequency drops significantly with more than 2 DIMMs (The memory QVL list for the Asus Prime B350-Plus reflects this), so if you're looking for the best possible speeds, you should get enough RAM for your future needs now, rather than planning to upgrade to 4 DIMMs in future.
- RAM speed drops even further if you go for dual rank DIMMs over single rank.

In general, RAM compatibility is still spotty, so you may find that while you might not get the rated speed of the RAM you purchase now, UEFI updates should improve the situation, but I wouldn't make a purchase based on that assumption.

Can recommend getting a 1070 - it's great at 1440p (60fps+ in most games), so I'd expect it to do even better at the resolutions you'd be gaming at. You might want to check prices for 1080s when you decide to buy, though, as they've dropped considerably since the 1080 Ti launched.

X370 motherboards tend to have more overclocking features than B350, and some can run PCIEx16 slots in x8/x8 for dual GPU solutions rather than x16/x4 (which will bottleneck the second GPU). I'd only suggest going for X370 if there's a good chance you'll need a second GPU in the future.

If you do end up going with B350, consider getting a mATX board - you lose a couple of expansion slots, but with the PCI-E lane limitations on AM4, you aren't losing much, and you open up more case choices.

If your games are mainly from Steam, you should just be able to copy your old Steamapps/Common folder (or folders) to the Steam directory in your fresh install of Windows. When you go to install a game you've already downloaded, Steam will detect that you have the files already.

It's a similar process with Origin games, but a little more finicky as you have to make sure you've quit Origin before copying the files to the new directory.

On a related but separate note, it's still possible to upgrade to Windows 10 for free, either by entering your Windows 7 key when you install Windows 10, or by upgrading using the assistive technologies upgrade path (assuming you use assistive technologies).

 

I think you misunderstood me - I assumed you have win 10 already - if not I recommend upgrading it(you can still get free upgrade if you say you are disable person - google it how) - if you upgrade for win 10 all your programs from win7 will work normally . What I mean if you have windows 10 and you plug this disk with windows 10 already instaled and all your data etc. into a new computer - you will normally boot into windows without any problem - win 10 will manage all drivers conflicts and upgrades etc. Example my friend recently changed his platform (motherboard + processor + ram) but keep the rest of his components the same he just - unplugged everything from his motherboard - took the old motherboard out - put new one in with new processor and ram and put cooler back on - plugged everything in again and win 10 started normally and managed everything fine.