I suggest you have alook at the product site
http://www.adobe.com/products/photoshopelwin/ the later versions do handle RAW files but don't impliment curve adjustments in the same way as Photoshop and might not meet your requirments. I don't think it does RGB layers. I started with elements (an early version) and moved to CS to get the fuller functionality. You could look around for an older version of photoshop as it's done curves/RGB Layers for many years but you might need to get CS3 if you want to handle RAW files from cameras launched in the last 12 months as they stoped supporting RAW in the older versions of photoshop (and elements) for the newer cameras e.g. CS which I have won't support RAW files from the nikon D300 as the newer RAW plugin that's required is not backward compatitable with older versions of photoshop (but I don't have a D300 either!)
Note: I haven't used the programs for astrophotography only 'normal' photography.