my 2 cents:
Third (or fourth or fifth) Big Easy. Pretty solid Cajun food.
There's a pretty delicious burger place in Plano called Ye Ole Butcher Shop. What might make it worth your while (as I'm sure you have plenty of favorite burger places in Atl) are the various game meats they offer. Every day you can get a great Buffalo burger and they have daily specials as follows (Monday to Friday respectively): wild hog, 12 point buck, elk, buffalo (not really special as they offer it every day) and steak sandwich. They also have homemade fried pies which can be pretty delicious.
If you go to Meshacks, in addition to the ribs and brisket, I recommend the hot links (bbq beef sausage). Like brisket, the links are very distinctly texas bbq so you could consider it. The only critical advice I have for Meshacks though is to ask for your sauce on the side unless you want your 'cue completely drenched.
Will you have a car? If so, you should peruse these boards for the best items in Dallas as it isn't too difficult to get from Plano to most other parts of the city.
BTW, i'll be moving to Atlanta in a month, can't wait to see what's cooking over there.
Poor Richard's serves one of the better breakfasts in the area, and their lunch is good as well. Best banana pudding in DFW imho. http://www.poorrichardscafe.com/
Fishmongers serves dependable fish. Their fried catfish is good, but their bread pudding is really good. http://www.fishmongersplano.com/
as mentioned before, the Big Easy is great....I think it's owned by the same people as Fishmongers. I like their po-boys and onion rings. Peanut butter pie is good as well.
For a good beer and good bar food, I like Fillmore Pub in downtown Plano. http://thefillmorepub.com/
Kelly's Eastside as mentioned above is a good place...they've somewhat recently expanded and added a bar area.
If you like tacos, there's a carniceria on the NW corner of Jupiter and 14th street that has the best tacos in Plano. You order and pay at the cash register, they take the ticket to the cooks. Coke from Mexico or aguas frescas to wash it all down. Taqueria La Paloma gets more pub, but the tacos from the caniceria have more meat, and are cheaper as well.
There's not much in the way of good BBQ in Plano. I haven't been to Meshack's yet, but that's probably your best bet for brisket. Otherwise, try Kelly's Eastside Diner in old downtown Plano (K ave). The brisket is ok but not great in terms of central Texas, but overall it's a good restaurant. On Mondays they have all you can eat pork/beef rib combo. Sheik's is an Arab diner that's just down the street. Good tabbouli, pretty good lamb.
The crawfish etouffee at Big Easy (just off Central at Park) is probably one of my favorite dishes in town. It's still a tad early for crawfish season, but I definitely recommend either this, or trying out Pappadeux if you feel the need to spend some more money, to try some Cajun food.
Jasmine Thai on Spring Creek is solid. I like the jungle curry myself. Little Sichuan and Umeko (for the Taiwanese specials) in the Asia World center on Legacy are also good. The JS Chen's there does passable dim-sum that you can order at dinner, it's better than their Saturday morning offerings.
The Fish Shack on 15th, also downtown, is pretty good for a shack. I go on Sundays for cheap oysters.
There are also some fancier places that people like, imo they're not really worth it though. Jasper's serves "backyard grill" food in an moderately upscale environment. I haven't been to the Naan Korean restaurant in the same area, but I've heard it's good. There's also a relatively new Turkish place there.
Yao Fuzi is a pretty popular fusion type place, serving somewhat Americanized and gussied up Chinese food. At least I think so, they used to have a more authentic menu but I don't know if any of that is still available or not.
Brisket at Meshack's has been consistently good from my two visits and overall I'd say the ribs are the best thing there. Very well could have just been that given day, but in any case, I noticed they particularly were in high demand. Mr. May's has also suggested the potato salad, will try it next time.
Downtown Plano has some good places as mentioned by luniz, Sheik's would be my favorite for that area. Past visits, I've shared the platter for 2 with friends, which can satisfy 3 or 4, it's a gargantuan feat. I like it since you get a diverse selection that includes hummus, several meats (lamb included, very good), falafels, etc.. Jorg's Cafe Vienna is also closeby and they're good for reasonably priced German food.
I usually go to Mr. Wok for big group dishes like peking duck or beggar's chicken, but it is also worth noting that the ramen has gotten better. Nice way to warm up during a cold day. Jack has turned down the sodium as suggested by the D Magazine review.
And on the inverse: if you get the craving for ice cream, Henry's is the place to go. Delicious homemade ice cream.