what you might gain:
* going from a 17" CRT (prob 16" viewable) to a 19" LCD (19" viewable) will give you a HUGE amount more screen space, which is a definite win
* LCD's typically produce much sharper and more readable text
* a newer LCD might have better colors/contrast than an old CRT, but that very much depends on the LCD and CRT
* LCD's take up far less desk space and weigh much less
* LCD's use less energy than CRT's, so you save money on your energy bill
what you might lose out on:
* changing resolutions: many LCD's look pretty bad at anything but their native resolution, so if your game doesn't support it or your video card isn't fast enough, you're might be out of luck
* you might get dead pixels (check the warranty and return policies)
* ghosting: older/crappier LCD's had ghosting problems... this is typically not a problem for newer LCD's, but check, or this could ruin your gaming and movie watching
* viewing angle: some LCD's only give you the proper (brightest, best looking) picture from a limited viewing angle
* backlight bleeding and poor black levels: many LCD's have issues with backlight bleeding on the edges of the screen and have trouble displaying rich black colors (since they are backlit)
my opinion:
* if your vid card has dual outputs, you can keep using your CRT in a dual monitor setup, and then you'll have TONS more screen space and the best of both CRT's and LCD's
* for $100, it's a worthwhile upgrade for the extra screen space, but be sure to weigh the pros and cons.