Philipp,
Sorry for any delay. Yes, using the Bold option vastly improves the readability, although I'd say that pushing up the font size by another point or two is desirable. Also, I'd like to make it clear that this vast improvement is only sensibly viewable in Internet Explorer on my dual-browser setup. In Firefox, for example, the entire layout of your homepage gets moved around and, as before, long strips get generated, with lots of blank areas around. I've tried putting Firefox's default fonts back, but that's made no difference. I think maybe the default fonts and default sizes don't actually return when you uncheck 'Use My Fonts' in Firefox.
At present, in both IE and Firefox, your webpage now, width-wise, occupies 10" of my available 16", which is certainly an improvement but, for my 20" monitor, is still wasting a lot of screen space.
Do keep trying to improve it. Check how things work out, using the Firefox browser (as well as IE), if you can. Then, if you settle on a good compromise, you'll be able to tell me which default fonts and sizes to use in Firefox. Note that, in Firefox, I set my minimum fontsize to 15.
Of course, you might very well be able to reach a very good compromise on this. However, in order for me to continue using Firefox for other completely different websites, I'll still need to use my customised Firefox font settings. As I said at the start of this whole discussion, the fault lies with all those other webpage designers; you're the first to take this seriously (and I applaud you for that). Your current provision of an Appearance option on the homepage is a good idea.