New disk in my PC and STEAM

Its likely that I will go to a cheap late tech parts for my PC like SSD and Motheboard/processor.
I have in STEAM :

1. ETW Ultimate edition
2. Shogun II and its Expansion plus varius DLCs and FLCs
3. Rome II WITH varius DLCs and FLCs
4. Attila TW WITH varius DLCs and FLCs
5. Few more games.

After re-installing STEAM and activate my Account do I have to find all disks of the games or simply STEAM "remembers" what I had installed and I can download it again?
If yes what is the procedure?

The games you bought from steam are in your steam library.

Just open the game library in Steam. All games you ever registered to your account will be listed here, free to be downloaded and installed again at any time.