is it possible to travel by high speed rail from Guangzhou to Chengdu? How much does it cost?

there is no high-speed rail service from Guangzhou to Chengdu, only normal ones, and the journey can be as long as 30 - 40 hours. so i suggest that you take a flight instead.

how about buying two tickets? one from guangzhou to wuhan, another from wuhan to chengdu?

is this possible? have they completed the high speed rail from shanghai (that goes through wuhan) to chengdu?

if it is possible, how far in advance should i purchase them?

yes, that is possible. The Shanghai - Chengdu high-speed rail will be put into service from July 1, 2014.
You'd better book 20 days in advance as there in only one bullet train from Wuhan to Chengdu and tickets will be sold out very soon after they are put on sale.