The xecuter series of chips are probably the most popular. However, a lot of those chips are a bit pricey and the features are totally unneeded.
I have a Duox2. It's a cheaper chip but works well. It's a dual-bank chip so you can flash the bios and don't risk destorying it if something goes wrong. Installation was pretty easy, but requires soldering.
Lots of information at
www.xbox-scene.com if you havn't found that site already.