Windows 10 is faster than Windows 7 on my single-core machine with 2GB RAM so you should be fine spec-wise. The message that pops up in Windows asking you to register is just MS marketing so there’s no need to sign up to it now (or probably at all) if you don’t want to – you’ll have up to a year to upgrade.
I’ve had Win 10 running on my main development machine for several months and as it stands I wouldn’t install it on an end user machine – I’m still getting a heap of problems including several main stream peripherals not working (and for some reason it’s just stopped recognizing my iPad), so I would hold off on installing it til a few months after release when hopefully the bugs will be ironed out.
I’m running Windows 8.1 on my day-to-day machine and it seems comparable in speed.
When Windows 10 is released you’ll need to do an in-place upgrade to get the licence, then you can wipe the disk drive and install fresh if you want a clean system, so installing on another drive to save your current setup isn’t really an option.