I am going to replace the motherboard, cpu and ram of my windows 7 pc (at home, this is not a server, of course). Am I expected to incur issues which could require to re-install the OS?
No, you shouldn’t have to re-install Windows, but there are a couple of things you will need to be ready for:
You will need to install new drivers for the different hardware.
When Windows was installed, it installed the drivers for the old hardware, so now you’ll need to install the appropriate drivers. The CPU and RAM won’t be a problem, but the motherboard contains numerous components, each requiring drivers (chipset, audio, video, network, drive controllers, etc.) Worse, without the appropriate drivers, Windows may not be able to boot enough to even prompt you to install the new drivers.
What you’ll want to do is to boot into safe-mode the first time after installing the hardware, then install the drivers there (your specific hardware may have different instructions, but this is typical for a new motherboard on an existing Windows installation).
Once Windows has the basic drivers ready, you can usually install other drivers in normal mode (again, your hardware’s drivers should tell you what’s required).
Just make sure that you have all of the required drivers ready and on hand (download them before installing the new hardware).
Windows may require you to re-activate. Again, this shouldn’t be a problem; you should be able to re-activate online, but if your Internet connection isn’t working (e.g., you haven’t installed the motherboard’s NIC drivers yet), you should be able to activate via a phone-call.
Will an image of the system taken with the old HW be useful with the new one?
It can be useful in case you want to put the old hardware in. Like I said, you’ll have to install new drivers for the new hardware, so Windows will be in a somewhat different state afterwards. If you have the space, then taking a backup is a good idea in case there’s a problem like the hardware is defective and you need to put the old one back in while waiting for a replacement.
Since the make of the motherboard and the cpu is the same (in my case, respectively, asus and intel)
The make of the components don’t matter, the models do. Unless you are replacing the motherboard and CPU with identical model numbers, it will require new drivers (and even then, it might still need new drivers due to different revisions, etc.)
can I update the drivers before swapping the hardware?
No, you can’t install drivers for different hardware and even if you could, your old hardware would stop working; you need those to remain installed until you shut down. (Technically, you can “install” the drivers in advance, by which I mean putting the files on the system so that Windows can find them when it needs to, but that depends on your hardware’s driver installers; some won’t proceed if they don’t detect the hardware, other let you install regardless.)
Is Sysprep useful/required for the smoothness of the process?
You can “slipstream” drivers into a Windows installation disk, but you have already installed it. SysPrep is generally for cloning a system like putting the same exact installation onto dozens of identical systems. This isn’t really your situation, so it’s not of much use here.
For your situation, your best bet is to download the drivers, unpack them, and have them ready in a location that will be available to Windows when it tries to install them. This means that they cannot be on a drive that requires special drivers for Windows to be able to access them (e.g., RAID); consider putting them on a flash-drive or erasable CD/DVD in that case.