Wenchy,
It doesn't surprise me that KR works under Wine, as I used to run KR and it worked until they added the cloud layer (which they have since removed). The issue is that I have a downspec Eee machine and I was surprised I could run KR in Windows of all (slow bloated) things. Netbooks were never intended run these sorts of applications.
There is a puplet specifically for the Eee (Project Ripple) that runs very fast, but for the sake of speed you give up functionality; I tried to get it run UO2D but I was lost in a sea of code that quickly grew beyond my ability to tinker and it seemed there were larger issues with the main Puppy distro. Ubuntu was/is a pleasant compromise. That being said in Ubuntu on the Eee, UO2D under Wine can be a bit choppy. I doubt Linux Mint would be that big of an improvement as it is based on Ubuntu. At least a big enough improvement to run KR on the Eee.
To answer your questions: Win7 is using 4.15GB and Ubuntu is using 3GB of HD space (not a huge difference) with the same programs and features loaded on both. I've never had a slowdown under XP with prolonged use as long as I'm doing system maintenance, so it will probably be the same with Win7.
I greatly prefer Linux for such features as tmpfs, and the fact it's full featured, fast, and free; but it will not run KR on an Eee without some tinkering way beyond my pay grade.