I registered at SX's ezboards on April 17, 2000. I believe SX's inception was sometime around the later half of 1999. I remember Arakon mentioning a while back that SX started out as a site about Sega music.
IIRC, SX went through at least 2 ezboards, the latest being this one. SX switched to IkonBoard 2 in April 2001 when, as megametalgreymon mentioned above, popups and ads started infesting ezboards. One year later (i.e., April 2002), SX upgraded to IkonBoard 3.
SegaXtreme has been around for.. well, christ, it was sometime in the summer of 1999. I remember "Iceman2k"/"Limbsaflyin" spammed the newsgroups about his site with Sega music and we've been good lovers ever since then.
I'd say late July/early August. SX was started around the time my Saturn site got up. Back then, it was just about music. Some roms here and there later, then ISOs after that. How many of you around today can say they actually logged into SX off of Iceman's own home server? Heh. I know of Arakon, myself, and some others.
Oh yeah, since no one (I don't think) said it: happy late 3rd birthday, SX. You ruined my life!!
According to Ice, he started his home server sometime in 1998. Of course, right now he's under ill influences (too much cough medicine, I'm sure), so you can take that as the truth or a lie.