This is the best and most widely used program for photo editing. It is expensive but it also will allow you to take advantage of all of the actions that are available. Most of these actions will only work with PS 6 or higher.
The full version usually costs around $599. It is possible to get a legal full version for much less than that. If you are a student or teacher you may qualify for an Academic package. This is the full package and can be bought at places like Diskovery $299 (do a search for Photoshop), CampusTech $269, Creation Engine $269, and Gradware $280.
I bought an older full version of Photoshop on eBay for $75. It is legal to transfer a license from one person to another. I then bought the upgrade to PS7 which usually goes for $149.
As you know one of the best things that a DSLR has over a film SLR is the ability to change ISO settings between shots. When we use the high ISO settings that are available to us we end up with a picture that has a lot of noise in it. Here is a link to an article that reviews many of the noise reduction options that are available to us.