I live within spitting distance of the main railway station, so at first glance, it seems like it would make sense to go there to buy a ticket and depart for Hangzhou next week (planning to spend Thurs-Sat there).
But as Hangzhou is south, it seems like departing from the South Railway station would make more sense.

So my question is this: can i buy an advance ticket for a south station departure at the main railway station, or do i have to go to the South station to purchase the tickets?
Or, is it just as easy to depart for Hangzhou from the main station and i should just get my tickets and depart from here?

Also, if anyone knows how to find reliable timetables online (that specify where the train departs from...), i'd love to see where.

Yes, definitely get the trains from the south (Meilong) station. These trains are faster, cleaner and also cheaper at 25/40 RMB for hard/soft seat as compared with 33/50 RMB with the main train station. Unfortunately you're not able to buy tickets at the main train station for Meilong station last I checked (couple of weeks ago) but this might have changed again, as I was definitely able to do it some time ago. If not, going to the nearest CITS/CYTS ticket office will enable you to get the tickets for an extra 5 RMB each, which should be cheaper than trying to get to Meilong station. Here's the timetable for trains between Meilong and Hangzhou (and vice versa), hope this helps.

Meilong--Hangzhou:
N521 0818--1006
N523 1006--1222
N525 1336--1524
N527 1614--1808
N529 1911--2105

Hangzhou--Meilong:
N522 0717--0912
N524 1051--1239
N526 1307--1518
N530 1622--1810
N528 1822--2044

BTW if you're taking the metro, please remember to get off at Jinjiang Park (Jinjiang Leyuan) station for Meilong station, and not the Shanghai South Station.