LoL! You will get PR from external backlinks regardless of the topic and niche of the donor sites! OMFG ...
You need what is called google backlinks. These backlinks are mostly authority links to google. Take popular sites like digg and stumbleupon, google will trace those links. Basically the higher the pagerank you link to the better as google trusts these sites more as they have reputation and some value. Now reminder that now all high pagerank websites are safe as some people tend to fake them out for money purposes but i think you get my point. You will see the more popular websites on google linked.
It takes time for google to actually give you a pagerank. Takes up to 6 months at least to see results. i suggest you keep building links and updating content on your site and not worry about pagerank.