And yet another 10 years later they find that those "cheap chemicals" cause cancer...

That's great. This combined with existing anti-retroviral therapies might actually extinguish HIV from the face of the earth in only a few decades, if used widely.

Spare the irony. For traveling, the outlined equipment is quite adequate, I know from my own experience (i.e. less than 2kg of equipment, which can be easily carried around even for hiking trips without tiring the back). If the OP was a professional fotographer, he would probably just resort to a hardened roller case with ample space for both the computer and the camera(s) with lens(es) and a tripod to boot. As for the financial aspect: If both the DSLR and the MBP are fairly up to date, their resale value is probably higher than the new price for the "light kit". Since the OP was asking for a solution and not for a bargain, the reasoning is sound.

A 17" MBP and a big DSLR just happen to be a bad choice for travelling. Go for a 11" MBA and a Panasonic or Olympus MFT camera instead. Almost the same capability at a fraction of the weight and bulk, and less expensive to boot. You can easily half your pack's weight this way.

the answer is right there in the OP: "I enjoy mowing..." Why do people eat too much and become fat? They enjoy it. Why do people drink to much an become drunk? They enjoy it. Why do people spend too much? They enjoy it. Mankind is not rational and there is no need to be. If the guy enjoys mowing his huge lawn, let him do it.

and in Soviet Russia, the law(n) mows you!

The HindenbErg, eh? I'd like that with my Cheeseberger in Pittsberg, before I lose all my money to a bergler, thx.

If your only choice is between XP (SP3, I assume) and OS X 10.6.x and your goal is to keep the total cost of ownership as low as possible, then I think the decision is a rather obvious one: OS X. If however it's an option to upgrade the entire network to Windows 7 and thus have a uniform environment, then you might want to research into this a little further. As for software, I can't really comment, but with the right tools, Macs are still more versatile than pure Windows machines, even if it means having a dual boot system with OS X, Windows.

We all knew there were Universes in Universes every since Men In Black came into theatres, didn't we? Old news, really.