If I were going to start ripping my CDs, I wouldn't use MP3 - I'd use flac. In fact, after I bought a pair of decent tower speakers last year I began doing it anyway (and bought a few DVD-As and SACDs too, though unfortunately SACD isn't really rippable).
Buying an external hard drive would almost certainly be cheaper on a per GB basis. You said microsd but did you mean SD? microsd is about as big as your pinkie nail and I don't think they are available above 32 or maybe 64gb now. I know a 128gb SD card costs around $100 - you could buy a 2,000gb usb drive for less than that.
If you are getting into the larger cards you have to make sure that your computer will be able to read them. I would think a PC could, but many portable devices will not be able to read the high capacity cards.
A 500 CD collection ripped to MP3 will probably use around 40gb. Just as a reference point. That is assuming 80 megabytes per disc - roughly what a disc 12 song album of 320kbps MP3s would take up.