Steele-Bike,
As one who just spent a bunch of bucks on a major upgrade, let me strongly encourage you to stick with 8 speeds. My upgrade was for different reasons, basically going from overall cheap c--p components, a couple of which were damaged or beginning to malfunction, to decent ones. In the process I went on up to 9 speeds because in my case it only cost a couple of bucks more. I would still be perfectly happy with the 7 speeds I had. I still used exactly the same gear combinations I would have used with the 7-speed, BUT I had put together my own 7-speed cassette with exactly the gears I wanted because that combination was not available in a 7. When I went to 9 it was so, as I say, I use the same gears. I think if you sit down and calculate the different gear inch combinations and compare it to what you would get with a 9 you will find perhaps ONE specific combination difference because the others will be duplications. Even that one will only be a couple of inches different which is almost imperceptible.

Now if there is a specific gear combination that you just have to have that you don't now, check the cassettes available at harriscyclery.com. They list the number of teeth on each cog. You can probably get a new cassette for $30-40 that has what you want without the expense of a full upgrade.
FWIW,
Raymond