My best friend recently had a surprise meal organised for her at Pizza Express (her fave, and almost everybody will find something to eat there, nice restaurant surroundings etc) which was nice, followed by a bar and then a nightclub.
Something themed might be nice - Mexican's tend to be fun and the environment normally very relaxed and suitable for a fairly big group. I've been to quite a few Greek/Mediterranean restaurants which are small independant and tend to have entertainment/music/dancing throughout the night which is also fun - I've found the waiters in these places to be very friendly, put a lot of effort in to entertainment and getting everyone up to dance at the end or join in with the traditional Greek dancing/smashing plates etc. The food is quite varied and usually does more traditional stuff as well as grilled meats and chips/rice.
An Italian restaurant might be nice, usually a little more upmarket and with pizza/pasta there is usually something everyone can eat. I've been to some with quite a pretentious/stuffy atmosphere where groups chatting or giggling quietly seem to be frowned upon and hurried out asap. Of course it depends on what they're like, but I've had a few experiences like this in independent Italian restaurants in London. Places like Zizzi, Ask or Pizza Express provide a nice balance to this if it's that kind of food you're after.
I'd personally avoid Indian, Chinese or Thai - hard to predict the quality of the food if you haven't been there, and are more acquired tastes, so not great if you've got older relatives who I find are sometimes more hostile to trying spicy or 'unusual' cuisine or anyone particularly fussy.
I'd go for somewhere with a fairly generic menu - like i said, a mediterranean/greek or a nice italian chain, or a nice but authentic Mexican (no Old El Paso kits and rubbery food). Somewhere they'll be accomodating of a larger group and possibly who have some music or entertainment - there are quite a few places in London which have clubs attached or turn in to clubs after a certain time which are really good (Roadhouse, Los Locos etc), not sure whether Norwich has the same sort of thing.
Cost obviously depends on who is paying and how much money you want to spend/anticipate everyone else will have to spend. I'd say go for somewhere mid-priced, £15-£20 a head is probably about average for the italian chains, for a starter, main and drink which I think is very reasonable. If the bill will be split, don't go somewhere too expensive or book tables at The Ivy if you think it might pose a problem for some people, as you wouldn't want them to miss out because they can't afford a night out somewhere expensive on top of a birthday present etc. But at the end of the day it's your choice, eat where you want to and I'm sure everyone will be happy with that choice, it's your birthday after all!