Depends on your computer. Generally, you only want to run one cpu-intensive task per cpu, but if you have a hyperthreading P4, then you would want to run 2 tasks to get the most performance from the machine.
All DC projects I've ever experimented with only use idle CPU cycles, but many tie up ram even when they themselves are idle; if you have plenty of ram, this won't slow you down in terms of using the system. If you have 256 or less, you might start to run short of ram depending on the memory use of the projects you choose.
F@H is a pretty cool project and the one that I run, but can use a fair bit of memory on large work-units; it can however be set to only run smaller units that don't require very much ram.