You can still get the Nintendo DS second-hand. I just talked to my boys (both still awake) and got the word form them.
difficult child 1 says the DS Lite is better. It's more compact, it's lighter and all the games should work (some don't, on the older DS). As for what games would be suitable for a 7 year old girl - pretty much any of them, both boys agreed.
The Wii - it's also Nintendo. It is not necessarily an alternative, because they are used differently. The DS/DS Lite is something hand-held which travels with the child. For example, this morning the Wii is something difficult child 1 was playing this morning, with our TV being the screen on which all the action is happening. Then we had to go out, so difficult child 3 brought his Nintendo DS, it travels in his jeans pocket. On the train difficult child 3 was playing his DS and had other kids looking on. There are other games that can be played as a group, such as Monopoly, etc.
Another feature of the DS is that you can connect to other people playing on THEIR DS's. Often I've seen difficult child 3 or difficult child 1 playing a game, competing with other players on THEIR DS, often on the other side of the room. If you're near an Internet hotspot you can even connect your DS to that and play online with other players, as long as they're also connected and playing the same game.
The Nintendo Wii can also connect to the internet for a lot of interesting services, including world news, weather etc. You can also play games with opponents from other parts of the world, if you choose.
difficult child 3 also plays a game that connects between the DS and the Wii. The game on the Wii is "My Pokemon Ranch" and it's downloaded from the WiiShop Channel as a WiiWare title for 1000 Wii points (A$10 retail, approx for us). It connects to Pokemon Diamond or Pokemon Pearl on the DS and you can trade your Pokemon between the two game systems (hand-held and TV).
Both the DS/DS Lite and Wii are Nintendo, plus I think Apple had a hand in the development, so cross-compatibility is something that seems to have been built in and is slowly being 'played with'. difficult child 3 is especially technology-savvy, so if ANYONE is going to discover this sort of thing, it's him.
easy child is not someone who likes computer-gaming, as a rule. But she and BF1 bought a Wii and she was so enthusiastic about it, she talked us into getting one also. The Wii is a game system that has the capability of getting you out of your chair and active. Wii Sports is brilliant; I've heard of people taking them into old folk's homes and getting the inmates active and having fun, playing tennis, ten-pin bowling, golf - all sorts of things. There is no way I can do a lot of those things; same with husband. But we CAN play, with the Wii. We've often set up a four-person game and had a wonderful family afternoon with the Wii.
Wii Fit is really good for getting your balance, core strength, weight etc monitored and under control. It's also something that is fun to do either as an individual, or a group.
With both of these game systems, there are different games for different ages and capabilities. Before you buy games, find out more about them so you can choose for her level. There are places online with good (as in accurate) reviews, plus talk to people. If someone you know has a game, see if she can try it out on their system first.
The first thing difficult child 3 got on his DS was the pet dog one, which was really good for his social skills and personal responsibility learning. The interaction capability of this is marvellous.
There are benefits to both game systems. They are different. The Wii probably is more of a family thing but can be an individual thing too. The DS/DS Lite - pretty much an individual thing.
A really great DS thing coming up (in the US - we already have it in Australia) is the Cooking Guide. It has over 200 recipes in it, an interactive cookbook plus instruction guide. It has voice command, so if you're up to your elbows in raw chicken you can talk it through the steps. It also talks to you, telling you what to do next (and no four letter words from Gordon Ramsay, either). You can select recipes according to what ingredients you have, as well as according to how easy they are, how long you have, what calories etc. It adjusts the recipes according to how many people you need to cook for and you can even put together a shopping list, so you take the DS shopping with you and tick things off as you get them. I think it will be available in November. difficult child 3 has cooked a number of recipes since we got this last week, where before he would be VERY reluctant to do much in the kitchen, not without a lot of reassurance.
We have a highly technically capable user of both game systems here, so if you have any more questions, fire away.
Hope this helps.
Marg