Ive not seen a video card with tv out that cant output to 15khz. Not just this, but 1024x768 non interlace 15khz as well (this varies card to card, but at least 800x600 is pretty much guaranteed). Lower than 640x480 though might require creating a custom screenmode (not difficult, both nvidia control centre and catalyst drivers have inbuilt tools by default for doing this).
PCs and associated hardware are pretty darn flexible for video work these days.
