I also have a laptop with Intel onboard sound, which works fine for some stuff like you said. I have two USB audio interfaces, a Edirol UA-4fx and M-Audio Ozone, which I use to record mono or stereo guitar/keys/vocals. I have never had a problem with latency, even when monitoring the mix after passing through Ardour and effects or rakkarack. I am able to run both USB devices in Jack at 128 frames and 3 periods/buffer, which gives me about 8ms of latency. That is not really noticable in my opinion.
A couple things about USB audio to be aware of, correct me if I am off here crew:
Only USB 1.0 is supported by Linux, because USB 2.0 is not standarized.
USB 1.0 only supports stereo audio in both directions.
ALSA has a page on their website of supported devices which is a good reference.
As per recommendations, the Edirol UA-4fx is okay. At this point it works except for 48 and 96 sample rates, because it adds some noise to it that the windows driver removes and has yet to be reverse engineered to work in linux(and Roland/Edirol hasn't helped out at all :/). But its 44.1 I/O, MIDI and Effects work good for its cheap price tag.
The Ozone i really like because it is a touch sensitive MIDI keyboard/controller with an audio I/O built in. All that connects through USB so you can play the MIDI into a software synth and send it out the Ozone to your amp/PA/headphones. I use it for live performance as it reduces clutter by not having more wires/stuff on stage. This one is a bit more expensive I think, between $200-300 US. I'm not sure the conversion rate to EUR.
This is a nice site as it is a customized frontend to zZounds.com to only show Linux compatible hardware based on the ALSA list. Personally I like buying from Sweetwater.com as they have better support and easy returns if something doesn't work like you thought.http://www.linuxstudiopro.com/
Happy hunting dude!