@martin > Firstly - there is no sound card on the DJM-800 or DJM-350. ;)
Second - It depends on what method you're going to be using to DJ with your gear to determine if the built in sound cards will even matter on the player. How were you planning on playing the music with the CDJ's? CD's, USB devices, Control software such as Traktor or Serato?
Third - There is a difference in the highest possible sound quality between the CDJ-850 and CDJ-350, but I believe that is by using either CD's or USB devices. I don't know for sure about the built in sound cards between those 2 models, but I'm guessing the CDJ-850 might have a higher quality one.
Just my personal opinion - I would base my purchase on more than just the sound quality of the product, seeing as they are both so extremely high, I have no idea who in the world would be able to tell the difference. I know it's not fair for me to say, but it's honestly true when you think about how many people you actually think would notice it. What is more important in my opinion, would be the headroom on the mixer, the quality of the EQ, quality of effects, and overall what you need to make your mixing sound great and take it to the next level. ;) Some may say you need the DJM-800 with the CDJ-850's, others are more than overwhelmed with their CDJ-350's and DJM-350. It's really what you think would suit your style and needs better. Don't forget that if you get a gig and want to take your own gear, you're going to have to lug all that heavy stuff! lol Just something to think about on top of it. ;)