@Slaninar
You have a good point about the wear and tear on a ten speed. I hadn't thought about that, but it makes sense as the parts are thinner.
I actually have no idea about the expected/average lifetime of the chain and cassette (I have not taken very good care of the bike in the past, which is a shame, now that I've begun to ride much more). Though even buying both a new chain and cassette once a month is cheaper than a pass on the intercity buses for a month. (And the wear shouldn't be that hard, even on a ten-speed?)
The bike is/will be stored warm and dry in the basement whenever it's not used, and there is public bike garages near school, so that should also reduce some of the tear on the parts.
The reason I want to switch to a double instead of a triple is that I never use the lowest ring (called a granny ring?), and after some time with the gear calculator from Sheldon Brown I found that by using a 50/36 double I would have about the same ratios/speeds I use now. (Another reason is that I like the looks of doubles. Which is not a very good argument when it comes to rationalizing stuff that are mainly meant to be functional...)
In either case I need to replace the crank/gear rings as many teeth are either worn down or bent out of shape. So now would be a good time if a should change to a double.
About using an eight (or nine) speed rear I thought about it, but figured that by using a 2x10 setup I would have a simpler and more flexible range of gears, which would be a positive thing as I could make many small changes depending in the flow of traffic. But please correct me if I am looking at this from the wrong angle here.
I can see how much of this this looks like I'm trying to rationalize spending lots of money on an old bike, but I value all input.
@blamp28
Thank for the input. I don't have the X9 readily available where I live, but seeing as they are listed as using the same technology as the X5 on SRAMs site there shouldn't be any difference, right?