HI,
alot of the preview of effects has to do with the software, and what effects. basic transitional effects should be real time preview.
a sepia effect could take awhile.
Adobe used to require a hardware card like Matrox inorder to do any decent preview in real time. now computers are so fast and adobe is better coded that most previews can be viewed real time.
Vegas seems to allow most again, Edius is funky with some of the codecs so again depends, Avid..well thats another story but most simple effects can be previewed real time.
one of the things i liked about Liquid most was its abilty to preview real time and allowing both the GPU (video card) and CPU to render whilst previewing.
the faster the system the better ability for real time preview. (cpu, video card is important here)
the faster the system the faster the renders. CPU and HDDs are more important for this, however some software gets help from the GPU on render.
basically an Intel core 2 duo or quad will do faster than real time renders (depends on effects used)
where an Dual Quad Xeon could render in 1/4 or less of real time. (assuming standard effects nothing crazy)
a core duo laptop will be less but could come close to the core 2 duo desktop if you have Esata external drives and a good graphics card.
generally 10-20% slower than same speed desktop.
Scott
ADK