Sounds like something is out of balance. Hard to notice at lower RPMs and once it reaches a Higher RPM then it appears to go away just that it is happening so fast that it is harder to notice.

Imagine a Tire that is not balance. I had a bad tire (wheel alignment problem) on my Civic. The car would drive Smoothe below 50, but I would feel the vibration in the steering between 50 and about 75 once I was above 75 it appeared to go away.
My guess is that since we typically wind out 1st and second that is probably why you feel it only in those gears. In Third 7800 RPM's you're going pretty fast (speeding) and don't even mention 7800 in 4th or 5th (unless you're at the track)

First thing and Cheapest thing to check is the tires. Make sure you don't have a nail or something stuck in the tire, Check the wear on the Fronts especially look for uneven ware. Check your Axels while your at it.

If Something internally is out of balance it can cause MAJOR problems in the future. Unbalanced Fly wheel can shatter or cause fatigue in the crank Shaft. Make sure the Crank Pulley is an OEM pulley as some of the After market ones don't come with harmonic balancers and can have adverse effects on your engine over time. (these are designed for race cars running about 7-10k miles between rebuilds)
It certainly may be nothing but I always like to expect the worse and take care of it before something Major goes wrong.

Good Luck :?