Yes, ALSA replaces OSS in the 2.6.x kernel series.
If you aren't using any KDE apps you should be able to remove arts without any problems. Even using non-sound KDE apps you should be able to remove it (although your distributions package manager might not want you to). If you do use a few KDE apps generally there's no harm in running it anyway, just uses up a few system resources, but generally not enough to worry about on modren computers.
Gnome seems to still use esd (at least the debian gnome-desktop-enviroment package still lists it as a dependancy).
Basicly both arts and esd are ways to merge several sound streams into one. This used to be a problem with older hardware that only allowed one application to access the sound card at once. Most current soundcards don't have this same problem, but arts and esd still have some uses (such as redirecting a sound stream over a network, providing a standard interface for sound programers, and other such things).
http://developer.gnome.org/doc/whitepapers/esd/
Gstreamer is something completely different. It's meant to become a standard framework for media. So, the idea that is if you want to write a music player or a video player or a sound editor or whatever, you just plug in gstreamer to handle the actual playback, recording, and whatever else. This way to support a new file format (eg. wav, mp3, ogg, xvid, mpeg4, etc.) you just need to write a new plugin for gstreamer then all gstreamer-based apps will support it. (Well, in theory. In practice this doesn't always work.)
Using a standard media framework also means every media-handling application doesn't need to re-invent the wheel, and the different applications should work nicer together. Gstreamer isn't yet official adopted by GNOME, but that does seem to be the general direction. Note that KDE or any other desktop environment could also use it as a media framework if they wished. (In otherwords gstreamer isn't tied to gtk or esd.)
http://gstreamer.freedesktop.org/dat...head/faq/html/
