Buffer management for SD card in SPI mode
Don't fear the FAT. It makes your job a lot easy.
The only thing you have to do is, before you start reading values at ADC, create the FAT 16 system and a text file. Then once you start reading, simply send the data to that text as simple byte.
I have done similar thing with PIC18F458 at a clock rate of 1Kb per second. Here is a link with sample code for same controller with generation of FAT system. This helped me a lot. I hope it would help you too.
This method was best for me as all the values were written in a text file and was easy to convert it in any other format or readable for other software as MATLAB.