GBIC vs crossover for connectiong switches

No.  That's why the gigabit ports are used for inter-switch connections.

1Gb/s half duplex =   1,000Mb/s theoretical maximum volume
100Mb/s full duplex =  200Mb/s theoretical maximum volume

Why are you stacking at half-duplex?

Theoretically no. 1Gb half duplex is still faster than 100Mb full duplex. However, I have to ask, why are the GBIC connections half duplex? Is that because of the transceivers? Anyway, the real test is do the transceivers introduce enough overead that a slower full duplex more streamlined connection would be better? Like I said, theoretically no.

GBIC connection will only allow half-duplex. 1000 Mb/s is great but if it is only allowing one way communication at a time, would 100 Mb/s then not be better in a high traffic enviroment?

No. If you want to look at one way communication you can break it down like this. The GBIC will allow 2000Mb to be transmitted 1000 - to and 1000 - from in 2 seconds. In the same two seconds the 100Mb full duplex will allow 200 -to and 200 - from.