Image processing on PAL/NTSC
You're probably best off using a dedicated chip to do the decoding. A quick trip to Farnell's website showed a few of these under CODECs/encoders/decoders, with digital output selected. This type of chip forms the front end of frame grabbers and some displays, with chips like Brooktree bt848 recording the data to memory.

That search found devices from Maxim, Texas Instruments, Analog Devices and Philips (NXP brand, apparently now bought by Trident who don't publish the datasheet).

It's possible you can build a lower cost alternative using the same analog circuitry an old TV would; a comb filter to separate into Y and C components. Digital comb filters are apparently patented. Intersil AN9644 might be one document to read.