Hey - not sure if you've gone to the casino already - but I think you should be ok with 800. I usually buy in 5/10 limit with 300-400 and doubling that for a 10/20 sounds ok. Haven't played 10/20 before so I wouldn't know if the game is that much different from than a 5/10 game.
Another thing I look at is I take a quick peek around the table at the size of the chip stacks. I remember playing a 5/10 once and I saw that only 2 people (11-seat table) had stacks above 300. So I bought in for 400. I know it doesn't matter for limit at how much you're going to buy for, but I wanted other players see that I'm adding a sizeable amount of money around the table and I'm very willing to use it.
Let us know how it goes and what you bought in for and what your thoughts are about your selection. I'd definitely benefit from the information you can provide!