If I delete 100-200 pages which are just user generated crap from my website will it impact my pagerank, or SEO ranking?
Anything and everything you do to your site will provide new and different information to the search engines the next time they crawl. The better question is will this make my site move up or down in the rankings. And no one can answer that for you. Bigger in and of itself tends to be "better" assuming the content is good. But the bottom line is this - check your stats logs. Are people visiting any of those pages? Are you making money from any of those pages. If not then do what you need to do to manage your site. There is a cost associated with managing junk.
Thats one! Check to see if you are ranking those pages. Deleting them should not have any effect on your plans as they would not be properly targeted. Review your plans first! Cheers
You can see if people are pointing to those pages (using google webmaster tools's external links). And check if they are landing on those pages / visiting those pages. If yes, you should probably retain them.
If the pages are indexed already, typically they will stay indexed for around 4 crawls or 6-8 weeks, which ever comes last. If people are visiting these pages via search, then it would be wise to keep them because all it is doing is giving people another way to get to your site. The only way that these pages can hurt you is if they are just taking up to much space on your server or they are riddle with bad outbound links. If you do purge the pages, make sure and submit your new sitemap to the major engines as quickly as you can. It will help speed the process.