I have never needed to replace any ethernet cables, the ones I do have have lasted a long time, plus I use wireless for my laptop anyway, I also found out that my particular wireless router is undetectable by the Nintendo DS for some reason, by actually trying to connect a Nintendo DS lite to it, the router is a D-Link DSL-G604T so don't get one of those to use with a DS, you have been warned, seems fine for other things such as computers though
The phone cable to the router was recently replaced with a shorter one, the original cable (which came from the ISP supplied USB modem) was packed up with the old router which is being sold very soon, and worked well, the current cable was unused until a few weeks ago and seems fine
although I have had problems with my connection they have been down to the ISP or British Telecom
no point buying a new computer if the cables break as they are very easy to get hold of