well there are a few ways... first is to just dump the backbuffer per
Nth frames into an open file.
next way would be to get a third party version of that.
i had a pretty damn good one about a month ago, i'll try to fish it out - it was called River Past Screen Recorder from Astrum, can't remember the website.
but you'll need a DAMN good processor to get it decently as well as enough ram.
i'd recommend atleast 1.3ghz (or atleast 800mhz above the game specs) ... and you'll need around 128mb per resolution double.
ie 640x480x32 = 128mb 1024x768x32 = 256mb etc...
also you'll need large FAST harddisk for realtime use - for anything under 5mins then keep around 20gb free and try to make sure its as fast as possible, so a DMA/100 7200rpm drive with a 2mb cache would be helpful - but the faster the better as this reduces the time it need to record the data in raw format (doesn't compress whilst taking pics, compresses once you finish)
it also records sound at the same time (you'll need to tag on another 300mhz for that on what you've already calculated, 500mhz if you want stereo)
but its definately the best thats really out there unless your willing to spend alot of money. Free version is fully functionality (almost cept for a few video compressors) for 30days and its only $20-30 to register