The intense interest in this latest Tate is not just to do with the fact that it has cost £134m, is constructed within Sir Giles Gilbert Scott's monumental Bankside power station by iconoclastic Swiss modernists Herzog and de Meuron, is about to open with a gruelling round of celebrity parties, and is in negotiation with a wealthy American collector, Kent Logan, over the possible gift of a chunk of his £100m Saatchi-esque stash of contemporary art. No: it is the fact that the collection on display has been, so to speak, jumbled up. No more do you find a chronological, art-historical sequence or division between artistic disciplines. Instead, you find much looser groupings organised under four main headings: "Landscape, Matter, Environment", "Still Life, Object, Real Life", "History, Memory, Society", and "Nude, Action, Body". Why four? Nittve disingenuously replies that it is at least partly because of the architecture of the building, which divides the permanent collection either side of its great central chimney on two levels, so making four physical sections. These are not on two consecutive levels, however: sandwiched between them is a floor of temporary galleries devoted to contemporary practising artists, one artist per room, with rough floors, rather like the studios they started off in. Only with much more expensive lighting.

All the new galleries are on the river side of the turbine hall - the inland side is still a humming transformer station. There is a cafe on the ground floor corner, a big shop on the turbine hall floor, a fine restaurant up on the roof, a coffee bar on the concourse outside the main galleries. Mammon is well served. You move up through the building on swish escalators, lifts, or ugly black staircases. The furniture is impeccably modern, mostly by Jasper Morrison. But the Tate's own modern collection - specifically the early modern stuff - used to be its greatest weakness. The Tate did not buy much early modernism when it was fresh out of the studio in the 1920s and 1930s, and has been trying to catch up ever since. If it had not had a disastrous director in the 1930s, a man named James Bolivar Manson who was inclined to get drunk and abusive at grand cultural soirees, who disliked modern art just as much when he was sober, and who was backed in his views by a very conservative board of trustees, who knows how our national collection would have turned out?

Now the Tate is displaying 60 per cent of its modern collection, rather than the 15 per cent it could manage previously, it is more exposed. It could be argued that arranging art thematically, rather than by date or movement, is the ideal way to disguise thin patches. Nittve is happy to admit to early worries about the stock when he took the job in 1998 - worries soon dispelled, he says. But it is not perfect now. "There are areas that could be strengthened," he says, "though the collection has proved much stronger than I had expected when I had only seen a fragment of it on display." Overall, he contends, it is one of the great modern art collections of the world. But yes, he says, early modernism is still a difficult area - not least because of the stratospheric prices it now commands, and which only American museums can afford to pay. Even so, he points to recent purchases of Miro, Mondrian, and Duchamp.