Congrats on considering moving to the most awesome city in the world. ;)
The Philly tech scene is small but dynamic and growing every day. For a sense of the scene, read Technically Philly and subscribe to the Philly Startup Leaders listserv [2]. The major clusters are Old City Philadelphia, home to N3RD Street and other related makerspaces, Center City, University City (near UPenn/Drexel), Northern Liberties as was mentioned. There are a bunch of suburban tech clusters as well, and the city is pretty evenly split between the two, I'd say.
There are tons of meetups in any given week on anything ranging from VR to Android to data. There are a couple of big conferences here every year including Emerging Tech for the Enterprise [4]. We also just hosted FOSCON. We have an active geek community in Geekadelphia [5].
Big companies (depends on what you think of as big): DuckDuckGo, Curalate, Linode is relocating here, Comcast is the number one employer in the region and is increasingly working on more cutting-edge tech projects (although obviously also still has all the features of a large corporate entity), Urban Outfitters down in Navy Yard. Then there are plenty of pharma , financial, and eds that make up the meat of the city's white-collar industry: GlaxoSmithKline, PNC, Temple, Drexel, Children's Hospital of Philadelphia, etc.
Where to live? A loaded question depending on what you value. Some people live in the city to be close to everything. The coolest neighborhoods right now are Northern Liberties and Fairmount, probably. Some people live in the burbs for the schools. Some burbs are closer and more accessible by train than others. Depends on what you're looking for.
Any more details, feel free to shoot me an email. It's my profile. I'm happy to answer questions!
[1] http://technical.ly/philly/
[2] http://phillystartupleaders.org/
[3]https://en.wikipedia.org/wiki/N3RD_Street
[4] http://phillyemergingtech.com/
[5] http://www.geekadelphia.com/