based on personal experience, i strongly recommend using a mac over a PC. The reasons have been extensively documented elsewhere. You can find a decent used ibook G3 or G4 for a few hunderd dollars on ebay. you will want to make sure it is running somethign close to the latest mac OS (OS X) to get the most advanced drivers. Of course, the price will depend on how much storage you want -- and how much storage you want depends on whether you will be storing your files in a lossless format (e.g., Apple Lossless). Based on your statement that you want "best quality" you will absolutely want to rip from CDs in lossless format. There are only a few places that I have discovered to download lossless files. Using an external hard drive is a cost effective way of expanding memory, but just make sure you get one that is very quiet and dependable. I have had mixed results with external HDDs, but the Western Digital My Book seems to be a decent product.
My setup includes a 120G Imac ===> Wavelength Cosecant USB DAC ====> Shindo Giscours pre. I could not be happier with how the Cosecant has performed. I have not listened to the Wavelength Brick (one step down the range from the Cosecant), but I am sure that it is a winner, too.
if you go with a mac laptop (not sure if this will work on a PC), make sure to check out Salling Software's "Clicker" application. You can download this application, install it on a a handheld device (like a Palm)and then use the handheld device as an itunes remote. the application works great.
Finally, don't believe the hype re: "high end" USB cables. So far as I can tell, any cable from a reputable manufacturer will do the trick.