Whoa there tiger....it's the middle of the night in the rest of the world. People don't generally get out of bed just to respond to posts on a message board. You're first post was at midnight New York time, which would be something like six in the morning in London, or something.
That said, why an I-PAC 4? It sounds like you just have a 2-player panel. Even if you have a dedicated 4-way stick on there you can wire it to the same spot on the I-PAC as the 1-player 8-way stick since they won't both be used at the same time. This is, in fact, generally preferrable as it means you can use either stick to control 4-way games. It makes it easier for guests to figure it out since the average person doesn't know what type of controller should be used with different games.
Anyway the rest sounds fine. That computer will play the majority of games (at least 80% I'd say) in Mame full speed, but there are a few that you would want a faster PC for. The hard disk is also too small to put a set of laserdisc games on like Dragon's Lair, but there are only about 10 of those games in all and you still have a couple thousand that will work just fine in Mame.
And drink less caffein.