Does it have to be ARM9? The beagleboard is pretty powerful for the price. $150 gets you a 500MHz ARM Cortex A8 (two generations newer than ARM9), 256MByte of RAM, 256MByte of NAND Flash storage, DVI video output, SDHC card slot, USB (Host & Device), audio in & audio out, and a bunch of I/O ports. It doesn't have built in ethernet, but you can use an inexpensive USB Ethernet adaptor.

Sparkfun also has an ARM9 board, but it costs more and IMO is less powerful (180MHz ARM9 / 64MB RAM / 512MB storage / no video out / does have built in ethernet though)

Another option is to buy a Nintendo DS...you get a 66MHz ARM9 + 33MHz ARM7 + 4MB ram + 2D/3D graphics hardware rendering w/ ~0.75MB of VRAM (which you can reuse as RAM if you need it all for gfx) two 256x192 LCDs (one has a touchscreen), WiFi, and microSD card storage (with an inexpensive adapter, under $20 these days), built in battery + charger, speakers, mic. Downside is you have limited interfaces to the outside world...either SPI in Slot-1, or the GBA Cart Bus in Slot-2. There is a free gcc port + libraries available, you just compile a program, and drop it on the microSD card, put it in the adapter, and put it in the DS. Retail price is $130 these days, you can get the original (bulkier) NDS for probably around $50-70 used on ebay.