Sure you can upgrade the 66-Mhz 68040i (stock CPU with no floating point) to a 117-Mhz PowerPC 603e if you can find a NewerTechnology or Apple CPU upgrade card--they haven't been manufacturered for about three/four (?) years though.
You can add memory too; maximum meory for the 500 series was 36-meg of RAM, (4-meg on the CPU card plus a "maximum" 32-Meg memory module.)
Finally you can upgrade the hardrive; check out
http://store.powerbook1.com/harddrives.html for 2.5" SCSI 6.4-Gig hardrive for $399.
I know of NO way you can upgrade the internal 19.2Kbps (Global Village) modem, though (i.e., kinda slow for serious web surfing.)
I love my PowerPC-upgraded 540c, but it's stuck with MacOS-8.0; the last system recommended for PPC-upgraded "Blackbird" Powerbooks (520-540 series.)
BTW, your 520 is over five years old now, your 520's PRAM battery should be replaced too--to keep the date in it current. This is not an easy task either--I recommend having your repair shop replace the PRAM battery.
By the time you've upgraded your 520 to a somewhat modern (but-retro) laptop, you may spend almost as much as for a NEW Blueberry or Tangerine iBook. I wouldn't upgrade if I was you.
I would never let my local Radio Shack work on any Mac or Powerbook--find a local Mac shop or try CompUSA (e.g., they sometimes have great Mac trouble-shooting techs here.)
Good Luck,
Terry J. Crebs
Lakewood, Colorado
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