The DS is a brilliant machine, but choose games carefully. Diddy Kong Racing, MarioCart DS, Kirby, Yoshi's Island, New Super Mario are all excellent.
Shhhhhhh.
http://www.dealextreme.com/details.dx/sku.1459
About £23 with a 1gb card. they allow you (with a big enough card) to play films on the DS and also use the machine as an MP3 player through the Moonshell OS. (You'll have to investigate).
I also have one of these,
http://www.dealextreme.com/details.dx/sku.2967
Less than a tenner at todays exchange rate, allows you to run GBA games, it has rumble and memory expansion (so you can use the DS browser etc).
They take while to deliver but they are by far the cheapest place.
The machine is really good and there is alot of thrid party software available (see ds-scene.net) You can even download a program called SCummVM allowing you to play all the old Monkey Island etc games on your DS.
There are also loads of hacks available such as a directional card so you can use the machine as a controller on Wii, or for your PC through WiFI etc.
Incidently if anyone knows where I can get a Transparent Shock case cheap please help (Im not paying £20odd quid for one!)
Thanks
Dan.