Hi. I'm new here.. this is my first post.
Welcome aboard!
I will probably buy all parts from ultimarc since they seems to be of very good quality (and andy is very helpful aswell). I have never had easy access to real arcade games so the x-arcade is my first longer usage of a (general) control panel and it has given me some thoughts about construction and feel in different style of games.
There is also the KeyWiz from
www.groovygamegear.com, but since your graphic mentions USB and I take it you're not sure of your PS/2 port, the I-PAC might be a better choice.
When I play fighter games with my friends the long travel of the x-arcade sticks (that are more or less a happ super copy from what i understand) is very nice.
However when I play most or at least many of the other games (mostly in mame) I have come to the conclusion that a shorter travel would make the controls nicer (not a 4/8-way issue) so I might want to build 2 separate panels because of this (and also to be able to play 4 player games). I belive that a stiffer stick might not be as fun playing fighter games but I can be wrong here since I am just guessing. You just have to correct me if I'm wrong 
I don't play fighters. Short travel is much better for older games like Pac-Man. I will try the Omni-Stik Prodigy from
www.groovygamegear.com, (short travel, medium spring) but won't have much to compare it to other than it should work tons better than my $2 analog PC gameport joystick.
YMMV.
Of the Other sticks, basically it's
Happ Super - Long travel, light spring, circular motion.
Happ Comp - Long travel, light spring, defined diagonals.
Happ Ultimate - Avoid
Omni-Stik, Euro-Stik - Short travel, medium spring
T-Stik - Shorter travel than Omni-Stik, stiff spring
J-Stik - long travel, light spring
(I'm not sure I would want a longer throw on my P1 and P2 sticks than P3 and P4, b/c it makes the third and fourth players at an advantage/disadvantage.) Depends how often you play 4-player games, though.
I also want the freedom to not have everyting wired up to the ipac and optipac but to be able to make alternative adapters to consoles and other applications.
I think the only concern here is you don't want the system to be connected to BOTH the console and the I-PAC at the same time.
I have come up with a system that would be like on the attatched image.
I want to be able to have the control panels on a distance from the ipac-optipac box so that 2 panels could be used as a 4 player system.
I eventually plan to do something similar.
If I use 3m standard VGA-cables meter cables . . .
This will work, but you can get DB25 10' cables for $1 or $2 each on
www.pricewatch.com. Look for RS-232 extension cables. Much cheaper.
. . . between the connection box and the control panels that would lead to up to about 300m of total cable length if ALL possible connections are connected. I wonder if that might make certain controller circuits to not be able to give enough power?? I havent calculated on this at all but it shouldt be a problem, or can it be?
No problems, each individual circuit is going through the 10 meter length. The fact that you have 28 paralled DC circuits has no impact.
Any ideas or suggestions or comments on anything that i have written here is more than welcome?
Post back if you have more questions.