...my 606, I just don't really use it anymore. I know it's sad but it's taking up space and I need the money for the other music purchases I will be making. So what would be a fair price to set for a 606 in excellent condition with a 512mb flash card and a brand new power supply?

I'd ask at least 300$, it's probably worth something between 300$ and 400$. Of course, with the economic crisis and stuff, you might have difficulties to sell it for 300$+. Having the 512MB card is good for adding another 50$-100$ to the selling price though, meaning I'd start with a 350$ price as your asking price and go from there.

I've seen an 606 go for 450$ a few months back. Some collectors might give you your gear's worth.