an average old CRT is about 1024 x 768. At least all the CRT's i have used or seen which was about 7. I think they could achieve higher res, but it will be so small that you cant really read anything.
i would choose a new monitor if you are going to game.
You could use the CRT to see how that performs, and if you dont like it, then upgrade.
I upgraded mine to a LCD with an DVI cable, and it looks pretty nice when i game, and it had a higher response time than the CRTs i had used before.