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.
The recommended minimum hardware for MIDI use is a 400MHz system with 64MB of memory. With an 800MHz system with 256MB of memory you should have enough power to sequence MIDI and audio with score editing and a small number of audio plugins.
More serious audio use will require more power: in particular, it's absolutely vital to ensure you have enough memory to keep all necessary audio components (synths, plugins, Rosegarden sequencer process) in memory at once – currently 256MB is really a bare minimum, as Rosegarden is a fairly memory-hungry application.
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.
For its user interface Rosegarden uses the KDE 3 libraries, so you will need KDE 3.x with Qt 3. You don't actually need to use the KDE desktop, you just need the library packages installed on your system.
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.
Please also consult the FAQ for more information.
Building from source code
In order to build Rosegarden, you will need a Unix-like operating system (most obviously Linux), the gcc 3.x C++ compiler, KDE 3.x with Qt3. See the FAQ for solutions to a few build and installation problems. You may also wish to build with ALSA 0.9 or better for sound, JACK (Jack Audio Connection Kit) for audio, and LADSPA (Linux Audio Developer's Simple Plugin API), for audio plugins.