I have just purchased the Canon ip5200 and am very happy with it (its bloody fast lol)

However, in my haste to buy it i only bought the PGBK black ink cartrige istead of the smaller black cartridge thinking that the PGBK was more expensive because it has moreink in it.

What is the point of two black ink cartriges and can i just use one?

Also are there good places to buy third party ink cartiges for it, at the moment i have got official ones.