How much do you want to spend?
One of my FAVORITE places in St Helena that's on the moderate side is Cindy's Backstreet Kitchen - owned and operated by the chef at the famous Mustard's Grill in Yountville - delicious, fresh American food with some Spanish and Mexican influences - my favorite dishes are the roasted quail and the mushroom tamales - I've never been disappointed there.
For something more high-end, Terra or Martini House in St Helena are both excellent. Terra has a bit of an Asian spin, while Martini House offers hearty dishes including fish, game, and meat - they also do a killer mushroom tasting menu.
For lunch, you can't beat Taylor's Refresher, a roadside hamburger stand in St Helena that is so much more than that - my favorite is the Ahi burger, served rare, with sweet potato fries on the side. For a sit down meal, Market in St Helena has comfort food with a local flavor.
I hope this helps - have a great visit in Napa- don't forget to bring a coat! It's been FREEZING here lately! :-)