First time in Philadelphia - pls help
http://chowhound.chow.com/topics/786247 is a great thread talking about some of the better restaurants in the Old City area.
Khyber Pass (gastro pub with a cajun flair and great beer selection) is highly recommended. If you like Szechuan cuisine Han Dynasty is good too. I am sure others will weigh in.
If you're going to be just in the Old City area, here's my recs:
Quick Slice of Pizza: Gianfranco Pizzeria Rustica
Good beer selection/excellent food: Khyber Pass Pub
Drinks/Italian: Positano Coast
Burger/beer: Standard Tap (this will require a short cab ride)