Here are the hardware sales figures for each system through the month of May 2007 according to NPD:

Nintendo DS - 423,150
Wii - 338,278
PlayStation Portable - 221,120
PlayStation 2 - 187,765
Xbox 360 - 154,932
PlayStation 3 - 81,604
Game Boy Advance - 80,554
GameCube - 10,728

Nintendo once again leads the hardware sales race in the United States, seeing its four systems responsible for 57% of all game hardware sold last month. Though down slightly from the previous month, Wii and DS numbers are still blowing the competition away. To put the sales figures into perspective, consider how much money Americans were spending on just Nintendo hardware last month: $55 million on the Nintendo DS and $84.6 million on Wii. Such numbers may be the reason why the game industry saw a 67.2% increase of hardware sales in May compared to last year at the same time.

via Nintendo World Report

Top Games for May 2007? Nintendo dominated with 4 of the top 5 titles with a PS2(!) sandwiched between them.

Pokémon Diamond (DS) - 331k
Mario Party 8 (Wii) - 314k
Spider-Man 3 (PS2) - 249k
Pokémon Pearl (DS) - 238k
Wii Play (Wii) - 227k
Forza Motorsport 2 (360) - 217k
Guitar Hero 2 (360) - 184k
Spider-Man 3 (360) - 140k
Command & Conquer 3: Tiberium Wars (360) - 138k
Guitar Hero 2 (PS2) - 131k