If you keep the sample size the same, the waveform till be less wide (to look at) but higher resolution.
If you take (just for easy numbers) 10 samples a second, and you have 1 waveform per second, you will have sampled the waveform 10 times. There are "empty" spaces between the samples, so you must average, or "dither" the samples on each side of the "space" in order to connect between the sample points.
Now at 10 samples a second, and your waveform is 10 seconds long you have 100 points = a very accurate representation of the waveform, but there are still "spaces" between the points that you have to extrapolate data between the points to show a nice waveform. You could also sample the same 100 points many, many times (like my scope does) to give an accurate representation.
Now, at 10 samples a second and you try to see a waveform that is one millisecond (.001 second) you may see it with 10 samples if you are lucky or you may not. Even if you do see it it won't be an accurate picture because you have at the most 1 point. You don't know if it gets larger, smaller, rings or whatever because you only got lucky to see it with one point.
This is why TEK and HP make so much money and have such cool products.
The quick answer is that if the number of samples is the same, you can make the waveform as nice (many points) as you want. You probably don't need 1,000,000 samples/second at 200 Hz but 1,000,000 samples/sec at 12 Mhz is not enough unless you do magic tricks with software.
Look up sampling theory, Nyquist theory or get some books on instrumentation to see how the scope works.
Look at this from Pickit2:http://www.sonsivri.com/forum/index.php?topic=6114.msg77694#msg77694
EPE used Cache Ram for the storage.
unzip it, look at his code. This is a beautiful instrument - very, very smart person