Is resigning now the responsible thing to do if I already know I'll quit somewhere down the line?

I recently made a switch to a new industry after 5 years of working in my previous field. There is some overlap in technical knowledge but the environment and work culture is not for me. I have an exit plan in place: I've applied to another job back in my previous field, which I have a pretty good shot at - I more than meet the requirements, I have a good reference from someone in the department I'm applying to, and I actually interned at the department back in college...though with these things its never a sure thing, and an opening in that department is really rare, so I'm sure competition will be fairly stiff.

My dilemma now is about when I should resign. My manager at my current job has been asking me to start looking a couple of longer-term projects, as well as to put together a list of equipment for the company to buy for use during the course of my work. This presents some issues - if I'm going to leave anyway the company should not be buying equipment according to my preference. It's quite a personal choice in this field and while the next guy who comes in will be able to use the gear I recommend, it might not suit his/her personal preferences. Me starting on a long-term project may also make things less smooth if I leave midway through and a new person has to pick up where I left off.

I won't be able to confirm if I get the job I'm applying for for at least three weeks. But that will probably mean I'd have started being involved in those long-term projects and buying equipment at my current company. If I don't get the job, I still don't see myself staying on at my current company for the long haul, but I'd probably stick around longer if only to prepare myself to start freelancing. So do I quit now?

edit: I should also add that beyond this sense of 'responsibility' I've also mentally checked out of my current job ever since I've learned of and applied for my hopefully new job.

usa jobs
usa jobs resume
usa hotel jobs
usajobs
usa jobs federal government
usa job in ksa
usa jobs
usa jobs login
usa jobs gov
usajobs.gov
www.usajobs.gov
usajobs.com
usajobs
usajobs.gov official