hey magic1,
As a curry fan and also someone who comes from one of the cultural centres of Asian Food (Leicester) I am a bit of a Curry Fascist and think there is nothing worse than a bad curry. Problem is there are probably 8 of 10 curry houses are shit and not worth being given any money let alone being allowed near a Tandoor oven.
A very good friend of mine owns 1 and has interests in 2 other asian restuarants (he doesn't let me say "Curry House"). And is lucky enough to have won the Best Asian Restuarent in Leicestershire Award for last 2 years and was runner up year before that. Not a bad accolade. So he knows his Onions (Bahji's)
His view is, don't order something that everyone has. Tikka Masala's, Bhuna's, Korma's etc are all mass produced especially once a restuarant gets more busy/popular etc. Try something different from the menu that will have to be cooked especially. Then your pretty much guarenteing (not 100%) to be better than a spoon full of the pot of Masala thats been on the stove for 48 hours.
Rice is also another interesting point. 9 times out of 10, its that which will make you ill. A lot of very dodgy places will leave rice in vats of water, boil it up, let it go cold and boil it again and again. This lets the bacteria grow in the rice and gives you bad guts.
Capsicum in the Chilli also makes you have the shits (as we all know) but can also make your bowel react and give you abdomen pain. So sometimes being ill, isn't a dodgey curry but just your bodies reaction to too much chilli. This can depend on the chilli's they use too...some have higher levels of Capsicum in.
For me, I really like a Pathia (Hot, Sweat and Sour) or a you can beat a good Jalfrezi. DOn't be afraid to ask for it to be made with Tikka meat rather than plain. This again, can mean its cooked fresh. Murchi Chicken is good, or even a Kheema (minced beef curry) with Chicken in good. I like a Aloo Matter (Chick Peas and Potatoas) on the side and a fresh Peshwari nan (the sweet one)...
All washed down with a few pints of Asian Beer (Kingfisher, Cobra, Tiger) or Japenese beers are good (Asahi, Tsing Chao). If you like wine with your curry, always go for Rose. White and Red have chemicals in that can distort the taste of the curry, or you distort the taste of the wine. Rose works best. A white shiraz or Zinfandel.
Once you find a restuarant that will let you order "not off the menu" and get something special stick with it. Give them a tip, and every now and again pop in and pick up the curry. They will get to know you and will be less inclined to give you crap. PLus they may then give you extra popadoms, bottles of beer etc for your "valued" custom.
Also, if you can try a take away that is also a good sit in restaurant. The food is normally a bit better and can be fresher as they are open for more hours and so food will probably be rotated more frequently.
Hope that helps!