Hey sounds like a fun project!
This is a pretty simple setup, just tedious.
1st: You're on the right track for the HDMI cameras, just ensure that audio is a enabled feature on them. I've seen them only support HDMI video to cut cost.
These are the current top 3 software suites. CyberLink PowerDirector, Corel VideoStudio Pro X4 and Adobe Premiere Elements. Also see what software your cameras come with, they may do the job if its simple.
2nd this is what I would use for audio: E-MU 0404 USB 2.0
or Focusrite Sffire 6 usb.
3rd for video cards: ATI is a excellent choice, just make sure you get Cards that support a crossfire set up or the equivalent process for Nvidia. These setups will get you all the performance you could possible want.
Lastly, ensure you get a motherboard that can support this setup. I prefer the Asus P6T DELUXE V2 or equivalent.
Of course you will need to get a true quadcore processor (intel hands down)
and a MINIMUM of 6GB DDR3or4 ram.
This setup will ensure this system will be up to date for years to come.
Oh, use windows 7 64bit OS or Linux if youre savvy and a minimum of 800w power supply.
Good luck, PM if you have any other questions.