Have you ever built an op amp circuit before? It is a lot more difficult than it sounds. You have to select resistors and capacitors for the correct gain, while maintaining the clarity of the signal. The chip doesn't matter as much, except to audiophiles.
The best bet would be to lookup the CMOY pocket amplifier. You then have to change the diagrams to be a five channel amp. (R, G, B, H sync V sync) The best chip would probably be a dual channel op amp from TI, they give free samples to "students".
I think the gain is around 5, but don't take my word for it. I am too afraid to do my own, and I'm an EE. For 15 bucks, I think it is worth the lack of effort. You will spend at least 10 in parts (bread board, resistors, sockets, wires, capacitors) Besides, sending too much voltage could cause massive monitor damage.