Theoretically you shouldn't have to do anything - just click the upgrade button, wait, and enjoy your new system. Windows will install its own nVidia drivers, but you can then update them via GeForce experience.
Steam works fine, just like all games I've tried since I installed first Win10 Insider Preview (well except the bug with MT Framework games on recent Insider Preview versions that I wrote about earlier, but fortunately it's fixable). You might have issues with some of the older programs and/or games (e.g. CorelDraw X4, which my dad uses, has some graphical issues with rendering main menu) but it's hard to tell which ones and what kind of issue before, well, trying them.
If you find that the system doesn't suit you, you can always revert to the previous version of the system within 30 days (unless you clear the system temp data that is).