i'm using a pair of mac powerbooks, one with live, one with modul8(video,synched via midi and controlled by live)
controllers are behringer bcr200 for knobs/buttons,
and a behringer fcb1010 foot controller.
for ultra-low latency i use an edirol fa-101 at 24/96k,
easily 2ms in/ 4ms out, 192sample buffer, even lower although i'm a bit nervous about artifacts(none at this setting)...also capable of 192k, 1ms in/ 3ms out at 192 sample buffer, possibly lower, although lots of cpu needed.(i also run vsti's, and trigger samples, so it's usually not192k, although as a dedicated looper it would be fine.
without using playthrough, the latency is actually not noticeable in creating/ triggering loops.
this is a firewire unit...with units which are pcmcia-slot(direct buss)
like the rme stuff, latency is even lower. with my old computer, my echo i/o card(pcmcia) did 3ms in/3ms out at a 256sample buffer.(no midi porton this unit, though).i currently use the midi port on the edirol to connect the foot controller, nothing else needed, ableton recognizes it. you configure your audio interface and midi input in preferences easily. the edirol is not cheap, around $500.00 u.s. for some in-depth analysis, go to ableton.com and do a search on the forums....lots of info! great forum, too...unmoderated, very open.
cheers
doug