I'll upgrade the tires on my bike when they wear out. My bike has about 60 miles on it, so I doubt that will happen any time soon. If I get some money and some guidance, I may upgrade the wheels. Anything beyond that for this bike is probably not worth it.

My bike came with Sora components. If I wanted to upgrade to 105 (highest level that makes sense for the frame), I would probably replace the cassette first, as I can't quite find a groove in the 8-speed. (I'm really strange, apparently--the bottom is quite comfy, as is the top, but the middle gears, not so much.) If I replace the cassette, I have to replace the RD. If I replace that, I need to replace the shifter. If I do that, I may as well replace the other one. If I do that, I may as well upgrade the FD. I looked up prices for new parts, and it comes to... $842, not including tax. If I include the cost of the wheels, I may as well buy a new bike.