Tex-Mex in Raleigh area??
I don't fully understand the distinction that you're trying to make. If you're looking for authentic Mexican food in a fun atmosphere, try a sit down Mexican seafood place on Capital called Vallarta. Huge menu, good service, and they serve drinks. We ordered several items and they were all good. It's been a while so I can't remember what was exceptional. My server spoke English, so that's a plus when you're with a group.
Let us know what you end up doing. I'd be interested to hear your guests thoughts about the level of Mexican food in Raleigh. I think that Mexican here is quite good for acity of this size.
I'll second Vallarta, if casual will do. It's not just seafood. They have all the usual suspects, plus a great selection of more "authentic" dishes. The fried quail is good, as is the posole. They even have agua de jamaica sometimes.
If you need more formal (and expensive) you can try Jibarra on Six Forks. I haven't been there yet, but everything I've heard suggests you can't go wrong there.
The best and most authentic Mexican food in this area is generally found in dumpy, hole in the wall taquerias, not the kind of place to take clients unless you know them quite well. The more presentable places, the "Els" and "Las", are your standard Super Combo tacos and chips places and are probably just what your clients *don't* want to go to.
if you want a more upscale place, try jibarra in north raleigh. chapel will posted on it a few months ago after a dinner he, i, and a few other hounds attended. authentic, but definitely upscale at the same time... maybe better for a client dinner.
I missed the word "client" in the original post. If you can swing it on your expense account, Jibarra is 100% the way to go. Jibarra is moderately expensive. They have great food, presented in interesting ways. Their take on queso fundido was one of the more imaginative that I've expereinced.
I don't believe we have any upscale Tex-Mex food in the area.
Jibarra is upscale, but it is not Tex-Mex. It is real Mexican food.
I have never been to Vallarta, so I can't comment on it. I would throw out Dos Taquitos on Creedmoor and Millbrook as a possibility, but not if there will be more than four of you -- they don't have any comfortable seating for groups bigger than four. Dos Taquitos is pretty Mexican, but they do have a Tex-Mex section on the menu. With the dark, eclectic atmosphere, it certainly is an experience to go there.