The Pumas' World Cup pool also features Scotland, Romania and Georgia, but they kick-off on 10 September against England in Dunedin and Contepomi stresses meeting Wales first in Cardiff will be crucial to their bid to beat the 2007 RWC finalists and 2003 champions.
"I'd say it will be our main game before the start of the Rugby World Cup," said the 68-times capped former Toulon outside half who will play for Stade Francais Paris next season.
"It's the key game for us in our preparation for the first game against England and it will be crucial for us, that we have the opportunity to play a great team as well.
"It is always a pleasure to play against Wales. They are one of the proudest rugby nations, I think it will be a party atmosphere and both teams will take a lot out of it in terms of preparation for the World Cup.
"It will be the time we will start to play rugby and to put all of our fitness work and preparation before that game into practice."
Contepomi is looking forward to returning to the Millennium Stadium, which will have played host to England's second RWC warm-up clash with Wales a week earlier.
The former Leinster player, who helped the Irish province win both the Magners League and the Heineken Cup before leaving to ply his trade in France, has also played in the English premiership for Bristol and is well travelled on the international scene.
He last visited Cardiff with the Pumas for another RWC warm-up fixture against Wales in the summer of 2007, when they suffered a 27-20 defeat in front of a 37,000-strong crowd, but he hails the Millennium Stadium as one of the best venues in the world game.
"I've been to a lot of rugby pitches and loads of stadiums, but the Millennium is one of the best, if not the best, rugby stadium I've ever played in," added Contepomi, who played at inside centre in that 2007 match and was one of the injury victims at Toulon last season which led to Wales' Gavin Henson signing up for his brief stint in France.
"I think it's a stadium made for rugby, it breathes rugby and it is a stadium where you have everything so close.
"You are at the heart of the City and in Wales, where rugby is a religion, it makes that stadium very special."
Tickets to the Wales versus England Rugby World Cup warm-up match at the Millennium Stadium on Saturday 13th August are priced £30 and tickets to the Wales versus Argentina on Saturday 20th August (both kick-offs 2.30pm) are priced at £25 and are on a 'buy one get one free' promotion.
Fans can also purchase their tickets for the Millennium Stadium clashes offer online at www.wru.co.uk/tickets or by calling the ticket hotlines 08442 777 888 or 02920 230130 or at local Tesco stores