[HKC]

Unless you have new converters, like the latest RME or Apogee, the
converters adds more latency than the actual soundcard if you can run on a
1.5 setting. If you have a firewire soundcard you probably have a
safety-buffer so the 1.5 setting may very well be a lot more. I find that I
can track very well with a 3 ms buffersetting on my old Hammerfall 9652. I
have never experienced any costumers saying that the latency was too big for
them to perform.
On the other hand I can easily hear the difference between 0 and 10ms when
held up against a drumtrack and I think everyone can, it's actually quite a
lot and can change the whole feel of the song but I find that much more and
issue while listening back than during reording.
If you by outboard guitar effects mean Sans Amp, POD etc I think the plugins
are just as good although I do like the hands on thing.
If you mean guitar-amp and stompboxes in my mind that's totally superior to
plugins but you need a nice amp, the right stomp-boxes, mics, preamps, a
place where there are no neighbours etc so very often it's easier just to
let the software do the work.
Of course during pre-production it may even be an advantage to be able to
change the sound of the guitar which can only be achieved when using
software.
Remember there are no rules, if it sounds right it is right.