Comics often celebrate milestones the way sitcoms do: with cheap, uninspired strolls down memory lane. ''The Amazing Spider-Man'' No. 500 is a cut above the usual self-congratulation, thanks to a compelling must-revisit-the-past-to-save-the-present plot by scribe J. Michael Straczynski (''Supreme Power''). Peter Parker's transforming encounter with a radioactive arachnid and Gwen Stacy's tragic plummet are grippingly evoked -- though the fleeting return of Uncle Ben is forced and maudlin -- while classic Spidey artists John Romita Sr. and Jr. pump up the nostalgia. The peek into the character's future may be ominous, but Spider-Man's standing as pop culture's most spectacular Everyman is warmly affirmed.