Factory speaker question

On a 2001 Chevy Camaro with the 500watt Monsoon system. What is a good replacement speaker for the front,rear, far rear speakers. I know the stock speakers are 2 ohm and most aftermarket ones are 4 ohm. Is there anyone who has found good replacement 2 ohm or can I use 4 ohm ones? I want to keep the stock amp and radio in the car. I just need new , better speakers.
Any suggestions.
Thanks

Image Dynamics makes a VERY good 2 ohm set. To use 4 ohm speakers, check out www.ls1tech.com 's stereo section. There are two thread about it "stickied" at the top.