The problem with recording video while also playing is that you are forced to split processing time between video compression and the game. Your best bet for this kind of extended recording is a real-time encoder that can take the load off the CPU, however such devices are not particularly cheap. Devices such as the BlackMagic Intensity and Matrox MX02 will perform such a real time encoding task, however they cost $200 to $500.
For best results, I would suggest outputting to an HDMI splitter and using a completely separate computer for recording with the capture interface installed on the secondary computer. This should also greatly reduce the space required since real time encoding hardware will provide a much higher degree of compression than real time CPU only encoding.