Look at these specs from the Windows Media HD page to be able to play 720p on a pc then compare to the N900 and i am sure you will have the answers.

Minimum Configuration
(to play 720p video)

Windows XP
Windows Media Player 9 Series
2.4 GHz processor or equivalent
384 MB of RAM
64 MB video card
DVD drive
1024 x 768 screen resolution
16-bit sound card
Speakers

Optimum Configuration
(to play 1080p video with 5.1 surround sound)

Windows XP
Windows Media Player 9 Series
DirectX 9.0
3.0 GHz processor or equivalent
512 MB of RAM
128 MB video card
DVD drive
1920 x 1440 screen resolution
24-bit 96 kHz multichannel sound card
5.1 surround sound speaker system.

Good luck is all i can say