Actually some of the larger capacity ones (8G, 10G) aren't really as big as they say they are. A lot of them may only be 256Mb. I don't remember how they did it, but when I was trying to find firmware updates for my generic 1GB MP4 player I ran across a lot of people who had bought larger ones from eBay only to find themselves stuck with a shitty 256mb mp4 player. They don't even play MP4 format files, it will come with a half English-half Chinese instruction manual and a little tiny white disk with a panda on it, that will have no instructions, so that you can find the converter hidden deep within the disk. The video quality is shit and I'm glad I got mine for less than my 512Mb MP3 player, otherwise I'd be pissed.
In honesty, just buy an iPod.