If for example you want to render a computer image (raytrace) like in blender this is almost always dependant on processor speed, its basically how fast it can do maths calculations. It has nothing to do with the graphics card and very little to do with the ram (unless you have almost none installed!). Vdeo cards are used for playback, especially whilst gaming. There are cards that do stuff with mpeg files, but these are again mainly for playback of DVDs and not for encoding (rendering to disk)

Rendering film footage is similar and is processor dependant. Just make sure you are not tying the computer processor up with other tasks whilst rendering. Anti virus software and messenger stuff sap processor power. If your rendering, disconnect from the net (to be safe), turn off antivirus and exit messenger services etc. then try again.

......Oh yes and if you have Vista installed, just get used to everything being so slow you wished you hadn't bothered upgrading!