Jailbreaking iPhone 3 GS?

I am a PC user and I am about to get an iPhone 3 GS 32 Gb in a matter of days. I want to jailbreak it. I want to install iPhone OS 4 on it but will it be possible to jailbreak it afterwards, if so how?
Also, how do you upgrade a jailbroken iPhone? Please be a little detailed in your description.

Big thanks in advance.
 
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