You could also download ROM Manager from the Market and use that, it's one click. It worked for me perfectly, but some folks have had issues where it would work. You can't hurt anything by trying. Try the Clockwork first, then if that doesn't take, then scroll down and try Flash Alternate Recovery.
I had to actually do that to my phone, but after I flashed the Alternate, I went back and did the Clockwork. I don't think it matters, but you need to have the Clockwork to use ROM Manager to install your ROM (CM6 or whichever you choose). Or else you will have to install the ROM manually using Astro, or some type of file manager, but it's no big deal.