A medieval city, in the desert, built as a planned national capital? you don't have too many historic examples to choose from, if that's the brief.

Here are some pointers:
You should know the terrain. You say it is mostly flat - how flat? where are the bumps? how big are those bumps?

This is a desert. Where do people get their water from? Is there an aquifer beneath the sand, so they can build wells? Is the city located near an oasis or a river? I've heard that in the deserts of western Peru in pre-Columbian times, people would set up nets to catch the dew that blew in off the Pacific Ocean - so if you're coastal you can get a small amount of water that way (although definitely not enough for 100,000)

Next question: do you know why this site has been chosen for the city? If you are the leader of a nation, you are going to be extremely reluctant to site your brand new capital somewhere so inhospitable, unless you have a very good reason (eg. defence, cultural beliefs, keeping control over your subjects). Furthermore, historically it is rare for a nation to emerge in a desert environment. A quick scan of Wikipedia reveals that the Arabian peninsula, the religious heart of Islam, became politically peripheral - the capital cities were Baghdad, Cairo, Damascus; much more favourable climates.

Remember, your city - no matter how important it is - needs to provide water and food to its citizens, be capable of attracting more people to live in it (being the centre of political power will bring a lot of people, but what other reasons do people have to settle here - will they have jobs? will they have security? if importing food is expensive, the cost of living is going to be high, so you want to be pretty sure you can find a decent wage before moving to the new city), have access to building materials (think how hard and expensive it is to get timber and stone into the desert, for example), not to mention dealing with the heat of the day. You also don't want your national capital to be too far from anything else - if a rebellion breaks out in some province, you might receive word too late to stop it.

Of course, it's your world, and you are welcome to ignore all those questions. They can just bring everything they need by magic, if that's what you choose. Or there can be more realistic explanations (like Egypt, which had the Nile to provide water, food, transport of building materials, communications etc.). It's up to you.

So, assuming you proceed with the city as you suggest, what should it look like?
You definitely don't need to render every house. Medieval cities rarely, if ever, had free-standing houses and buildings. More often than not, you'd share at least one wall with your neighbour. There's nothing particularly modern about apartment buildings, either. The Roman insulae, for example, was basically just an apartment block. Of course, tall buildings need to be light, so you're getting back into the area of building materials. I started a map a while back that tried to represent the way different buildings would get packed together. The post is here: http://www.cartographersguild.com/to...sque-city.html.

As for important buildings, Waldronate is right - cities are often clustered around areas of power - from the sound of it, your city will have a prominent palace. But remember, most of the ordinary people will never have a reason to go to the palace. They'll be more likely to congregate around a plaza/forum (as W. said), a temple or temple complex, or a market. Having said that, your city might not have many of those things visible - who wants to spend their day in the hot desert sun? The markets, forums, plazas will all be roofed or covered up, as in the Isfahan bazaar.

The stadium idea would probably also have some way of sheltering the crowd from the sun, but it can be an impressive public building nonetheless. But do they hold games/fights/tournaments every day? That would get very expensive. It might be an important building, but that doesn't mean people congregate there regularly.

Lastly, the layout of the city and the road network. The idea of big avenues and grand vistas is quite a new one - it was essentially developed during the European baroque (~17th-18th C). The Romans and Chinese (and probably others) also had major avenues, but not for the same reason - the Romans, for example, used them to march soldiers down.

That aside, follow the suggestions that others have made - look at maps and satellite images of cities for inspiration, but be sure what you are looking for. If you want examples of desert cities, look at Egypt, Africa and the Middle East.

If you want planned cities, Waldronate's link is pretty good. My suggestion - steer clear of Washington DC (no offence meant to Washingtonians). Brasilia has an interesting design from the air, but it is much bigger than your city. You could try looking at Palmanova in NE Italy - it's a planned town from the Renaissance era, but it's population is only about 5500.

The best city to look at is Canberra, Australia (yes, I'm biased). I don't think Canberra is very well known internationally, but it is still worth checking out:
- It isn't very large (about 300,000)
- It isn't very old (100 years exactly; although most of the city was only built in the past 50)
- It was planned as a national capital, and has the things you're looking for (avenues, and important buildings)
- It's design is more interesting than a simple grid (sorry again, Washingtonians)
- Canberra may not be in a desert (we're mostly scrub and bushland), but this is a dry continent
- If you look at Canberra, look carefully at the central area - you'll find that there are straight roads, twisty roads, and combinations of the two, which might help you solve that particular problem.

Well, I only meant to give you a couple of tips, but looks like you got more than you bargained for. Hope all of that helps. I'll keep an eye on this post, if you have any more questions/clarifications.