Hi
I think it will make a worthwhile difference. I'm not a higher bitrate bigot, keeping two copies of all my albums, one in 256k (mainly for use on my phone / iPad), the other CD quality for my main system. I also listen to Spotify at 320 Kbps on the main system (Mac Mini, rDac, Ayre, Harbeth) a lot and it sounds good. For proper listening (ie without distractions) I do much prefer CD quality (or higher) though.
Reripping can be a big job (took me 3 months) so before you commit to the time and expense why not rip a few favourite CD's to your hard drive and compare the two. Easily done and should say whether its worth it FOR YOU. if yes you might also want to check you've got the PC optimised as well - using the right drivers etc. I found that made a difference too. I'm now comparing Pure Music and Audivarna on the Mac and finding quite big differences (down to use of Integer play I think). Apologies if I'm teaching you to suck eggs on the PC side of things.
n.b. you should be able to stream from a external hard drive. Although I think ( stress think) that the Airport Express limits you to CD quality. Not a great problem I would have thought.