One pitfall not already mentioned is that simply combining the R-G-B and
sync signals coming out of a VGA card won't produce a US standard NTSC
signal. A US television signal (NTSC compatible) uses a 15.750 kHz
horizontal sweep rate. The lowest VGA resolution (640 X 480) uses a 31.5
kHz sweep. Higher resolutions use higher sweep rates. This scan rate
incompatibility may complicate your project. Of course, there are some
specialty video cards around that have an NTSC video output, in addition to
VGA. Those cards have an on-board scan converter.

Good luck,

Roger K6XQ