If your looking to capture just "screenshots" aka stills, then I'd recommend Xbox Image Grabber. Though, getting that little app to run will take a bunch of research on your part. You'll basically have to add debug functionality to your retail xbox. Not terribly hard to do, but there are specific files you need to find
on your own, and if your not familiar with applying a debug bios to your xbox then this can turn into one heck of a challenge. Plus there might be limitations on your drive size in support of partitions over 137GB's.
Though if you get your debug functioning, then XIG will take about the clearest screenshot offered by any app. Here's a link to a thread that will provide information about getting XBI up and running, most the information is rather vague so I suggest you to read through the entire thread and try to piece it together -->
XIG Thread LinkAs for a TV Tuner card, those are great for capturing full motion, which could come in real handy. Though the video captured by a TV tuner will be degraded, and when you compress it to shrink the file size, even more degradation will occur. As for a particular video card to buy, well I'd say do some research, and look into one that provides a S-Video input, which will allow you a bit more clarity compared to a composite feed.
I have a tutorial in my sig that details the various types of methods you can use upon setting up a TV Tuner capture card for xbox output.
Just to give you an idea of capture quality, I placed a comparison photo below detailing screenshots captured with Xbox Image Graber and a TV Tuner card.
