The New iRiver H120 is certainly the most established 20GB iPod killer on the market, but it'll set you back almost as much cash.
The Rio Karma is also a safe bet, it's an amazing 20GB player, and it's about a year old, so you can probably find a great deal on it.
There's also the Philips HDD120, what it lacks in stability it makes up for with style. It's easily the best looking portable player out there, and firmware fixes supposedly remove all the bugs which plagued earlier Philips players. The only drawback would be the IDV tagging, you must IDV tag all audio files, it doesn't sort anything by filename.
The new Creative Zen Touch has had very good reviews, but Creative has an interface that's really hit or miss, depending on the individual users. It gets stellar praise and many many insults at the same time.
Don't bother with any of Sony's products, they force users to encode music into their ATRAC3 format. What kind of mp3 player doesn't play mp3s?