The higher the fire resistance in relation to your overall hit points, the better. How many hit points do you have (what's your health)? You can use that to gauge whether 800 is good or whether you could use more fire resistance.

For instance, my vampire has about 1400 hit points. I have 900 flame resistance, and in my opinion it's not nearly enough for a vampire. I still get burned to a crackly crunch on a regular basis.