Hardware and System Requirements
Rosegarden is a multi-purpose application whose hardware requirements differ markedly depending on what you're using it for. Editing, playing and recording MIDI and editing score are less resource-hungry than recording or playing audio. The use of multiple audio plugins or soft-synths will especially impact system performance.
System hardware
As this is a Linux application, the first requirement is for a PC capable of running Linux.
Any modern desktop or laptop and most netbooks should have sufficient processing power and RAM to meet Rosegarden's minimum requirements. However, Rosegarden is fairly memory-hungry, and can demand a lot of processing power, so the more you can throw at it, the more enjoyable your experience will be.
You will get far better performance from Rosegarden on any hardware if you use a Linux kernel that is optimised for real-time applications; the best way to achieve this depends on your choice of Linux distribution, but for many distributions the easiest thing is to look for a "multimedia repository", "multimedia distribution" or "audio overlay" tailored to your distribution and install a low-latency kernel from there.
Audio hardware
Rosegarden requires and uses the ALSA sound drivers to drive your soundcard. The Rosegarden project itself has no control over whether a card is supported or not: it depends entirely on whether the card has ALSA support. To find out whether your hardware will work with ALSA, look it up in the ALSA Soundcard Matrix.
Of course, if you're using an external MIDI device (sound module or keyboard) you'll also need the appropriate leads and cables.
Software
Rosegarden runs on Linux, so you'll certainly need that.
Rosegarden requires the ALSA sound drivers for audio and MIDI support, and you will need the JACK audio server if you want to do audio sequencing, although it isn't necessary for MIDI.
If you intend to build Rosegarden from source, you will need to install a number of libraries. See the Get Dependencies wiki page for a list.
Please also consult the FAQ for more information.