The "older" alien race (which one of them was found dead on the derelict ship in the first alien movies) is called "Space Jocky".
The presumed fact is that they created the alien race to serve as a weapon, but apperently they (Jocky) couldn't control them (aliens) so they decided to take all the eggs that they had and take them away somewhere (with the derlict ship which appeared in the first alien movie).
Apperently one jocky (the one found) was infected with an alien contamination (which explains his burst cheast as seen in the movie) which caused the ship to crush down on LV-426 (not sure about the # of the planet). WY (Wayland-Yutani, the "real" name of the company in the aliens movies) learned about the derlict ship and the alien "weapon" spices, so they sent the Nostromo to go and fetch an alien sample (as seen in alien 1).
AFAIK, AvP first appered as an atari 64 game (you can learn it for sure at
www.avpnews.com). Afterwards was a comic book serise and several books (along side several other games for various machines), when in 1998(?) came out the first AvP game for PC (and it had an addon and a sequal, which also had an addon).
The idea to combine both universes came up from a scene appeared in the second predator movie, which let us see a glimphs of the predator's trophy room in his ship, and there was a skull simular to the alien creatures. Rumuer has it that some prop guys on the pred movie set added that skull as an inside joke, but luckly for us, someone saw that and thought it was a good idea
.
There's a lot of other information about the aliens, the predators and the AvP series on the net, which is more accurate, but this (might) sort some things up
.