Yes, you can flash the radio without it hard resetting your phone and screwing up your ROM. It probably couldn't hurt to try and see if it gives you a better EVDO signal. If it isn't any better, you could always just flash back to the radio you had...or try another one listed
here
Hope this helped ya out, and if it did, don't forget to hit the green "Thanks" button in the lower right corner
-Keir-