I am going to be in Houston over Easter. Is there any particular gun/outdoor store I should have a look at?

Collectors Firearms (they have a website), and Carter's Country should keep you busy.
Enjoy!

Collector's Firearms will more than keep you busy. Their website doesn't even come close to doing their inventory justice. It usually takes me several hours to look through their racks. They have that many guns. CF rules! You could also check out Fountain Firearms. They have a website too. Collector's Firearms is very browser friendly (in store). The majority of their guns are racked right out on the floor. There are relatively few behind the counter. You can fondle as many as you want. They also have a lot of C&R and antique guns.

If your looking for Custom Rifles try MG Arms. Owner is Kerry O'Day. And as it would be it is right across the street from Carters Country. This will all be out close to the Houston Intercontinental Airport. Within a 10 - 15 minute drive.

I wouldn't reccomend buying anything at Collectors or Fountains unless you just can't find it anywhere else. Their prices are highly inflated. Collectors is a very nice place to visit. Fountains used to be good when Jerry was running the place. Carter's is excellent for modern guns and you can sometimes find something interesting in their used gun section. Carter's has four locations.

You could also go to the Bass Pro Shop in Katy on the west side of Houston.

I don't know how much time you have but Briley's in Houston (the choke people) have a nice selection of used shotguns. Lots of English doubles last time I was there.