After reading this forum and doing extensive researching on forums here,
I finally purchased a Main Lug, squared model QO120L125GC. Now I need
to figure out where to install it. This box dimension is 14.25(W)x20.92(H)x3.75(D) (in inches).

Please see this jpg photo of my basement: http://i229.photobucket.com/albums/e...0/P1010052.jpg

This basement doesn't have much room for expansion, and space is
very tight around the main panel. The Red oval on the right-hand-side of main,
is where I would rather install the main log.
However, as you may be able to see, within 17 inches of
front of this area, there is the docks for our cooling system.
If the red oval area doesn't meet the NEC, how about the blue oval area
on the left-hand-side of main. I can remove the network switch, and
install the lug there. One thing to note: The main circuit bottom of the box,
is about 4 feet off the ground. Which means the network switch that you
see in the picture is about 6 feet off the ground. Does NEC code allow
me to install the main lug, in the blue oval area?

Next picture, an alternate location:
http://i229.photobucket.com/albums/e...0/P1010049.jpg

The issue with this, is that, the main circuit is about 2 feet on the
left edge of this picture, i.e. about 17 feet away, if one was
to bring a cable in a L-shape to the main circuit. So the distance of
being away from main circuit becomes an issue, which I hear would
be voltage drop as by product of longer cables from main lug
to main circuit. Another issue, is that, this is the furnace fire ceiling
protection area, would NEC code have something against,
installing the main lug around furnace?
Yet another issue is, how would I place the wire from main circuit
to main lug, and wire the basement, with the fire ceiling in place?
The joist you see, is the main support beam for the floor above,
so I rather not drill into that beam.
I hope there are good ideas for this. Thanks a lot.
-Ab

The first picture with the ductwork and copper line is out of the question.

A panelboard needs 36 inch clearance in front, and 30 inch wide clearance, not necessarily centered on the panel, can start from either edge.

Voltage drop is not an issue with 17 feet. With the proper ampacity rated cable, 50-75 feet would be fine.

As far as the ceiling, it is only sheetrock. Either staple the cables on the surface, which will be difficult to do and look like hell, or remove what you need to wire above the rock.

If the joist you point out is one of many every 16 inch on center, this is typical framing and drilling is common, as long as you stay in the outer thirds of the span.