Gabbidon, 25, and Collins, 21 both made a combined total of 302 appearances for Cardiff and also have over 20 full caps for the Welsh National team.
Alan Pardew said of the talented pair, “Danny comes to West Ham United with a lot of experience for a player of his age.
“At just 25-years-old he is already a key player for the Welsh national team and has shown a lot of maturity in his performances.
“Pace and agility will be his major assets as Danny makes the transition to the Premier League. He has already shown at international level that he has what it takes to perform at the highest level.”
“James is an excellent prospect. He is probably lacking experience as he joins the Club, but do not underestimate his potential.
”At 21 he has already played for the Welsh national side, and with the signing of James and Danny we now have five of the best young centre-backs in the country.”