Lionel Messi is now the all-time leading scorer in World Cup history with 18 career goals in the competition, adding yet another milestone (well, one of four new records he reached during Argentina’s 2-0 win over Austria) to his incredible career.While Messi was making history in Monday’s match, he was wearing a variety of special patches on his shirt, including a World Cup Legacy patch (worn by players appearing in five or more World Cups) and a Golden Ball Winner’s patch (worn by players named the best in a previous edition of the tournament), both of which will eventually be put into trading cards once Topps takes on the FIFA license in 2031. Although that is still a long way off, the cards bearing these patches worn by Messi on such a historic occasion will likely be worth millions of dollars once they’re made and released — perhaps even tens of millions, depending on the state of the now booming trading card market and demand for Messi items at that time.Although soccer cards are still far behind baseball, basketball and American football in overall collector interest, their popularity and value is rising, thanks in part to the excitement currently being driven by the World Cup in North America. Two Messi cards have sold for more than $1 million each within the last year (and a third came close at $960,000), but the card bearing his Legacy patch from Monday’s match in particular could one day dwarf that. Premium value is being placed on extremely rare or one-of-a-kind cards bearing patches worn by athletes in key moments, and there aren’t many bigger individual achievements than this.But what if you’re looking to build up a Messi card and memorabilia collection right now? Here are a few key cards to know:Most prominent rookie card: 2004 Panini Sports Mega Cracks #71(Photo courtesy of PSA/eBay)Like Michael Jordan’s 1986 Fleer #57 card, this is currently Messi’s most well-known and valuable rookie card. Those three sales in the $1 million range mentioned above were all for examples of this card in top condition, with the high mark being $1.5 million, which is a record for any soccer card to date. This card was printed in much smaller numbers than Jordan’s ’86 Fleer card, for example — there are less than 2,500 total graded examples of the Messi Mega Cracks, with just 20 in Gem-Mint PSA 10 condition, whereas there are more than 58,000 total graded examples of the ’86 Fleer Jordan, with 334 in PSA 10 condition. That illustrates how limited the supply of key soccer cards are compared to the more widely collected sports and how quickly an increase in demand could skyrocket the value of such cards.Note that there are other Messi cards from various 2004 Panini sets, but this one stands above the rest.Most undervalued rookie card: 2004 Mundi Cromo Liga #617Who is Luis Leonardo Messi?! (Photos courtesy of PSA/eBay)Although the aforementioned Panini card is widely recognized as Messi’s primary rookie card, his Mundi Cromo card was reportedly the first to be released. But because Panini has global brand recognition and Mundi Cromo is a far lesser known Spanish brand, the latter has been overlooked by collectors despite its significance. This card sells for far less than the Mega Cracks, with a record sale in a PSA 10 grade of “only” $102,000 at the same time as those seven-figure Mega Cracks sales last year. There are 47 examples of the Mundi Cromo card in PSA 10 condition but only just over 2,000 total graded copies of the card.While it has no bearing on the value, what’s fun about this card is that it shows Messi wearing Nike cleats before he became Adidas’ biggest athlete on the front and, hilariously, the wrong name — Luis Leonardo Messi — on the back.