I'm interested, too.
Using Jack, Hydrogen, and Ardour I got my latency down to 11.6ms (frames 512, sample rate 88,200, M-Audio card). You can run Jack in real time but you may need to gksu to get it. That's what I do for Hydrogen, but I rather not run as root for these apps.
Here is some info on a zero latency kernel https://wiki.ubuntu.com/RealTime
I saw some other info but can't locate it right now. I recall it involved recompiling the kernel with the Linux Security Module (LSM). This allows real time access without being root. It looked more complicated than my experience is ready for.