cheapest?
but I really like my MOTU UltraLite it only has 2 mic pres, but can handle more 6 other trs line ins + one SPDIF-for 2 more channels its solid and glitch free.- firewire in and out- bus powered
I'd stay away from USB interfaces for doing multi track- unless that's your only option
you might ask why not go for a 4 channel recorder or a new 6 or 8 channel standalone the new Tascam 8 channel has promise!
I have mostly gave up on dragging my iBook and Motu and battery around
mostly do 2 track any way - unless I'm at festies where I can get power and patches and also bring an external firewire drive to record too.
I have and hated my Presonus Firebox it had more issues and glitches messed up more recordings than it made - a waste of money- but I got it right when it first came out. I know some who run it with windows units and they have good luck. I can use it as a standalone mic pre if I am ever doing any FOH work and it works fine for that.
MOTU is designed & built for Mac's
will you ever run 4 mics? do you need more than 2 mic pres?
the MOTU traveller is cool too.
good luck
--Ian