A primary reason to use Photoshop over the gimp, quite apart from its greater capabilities as an imaging tool, is the enormous community that you instantly plug into. Photoshop isn't just a graphics product, it's a vast infrastructure of users, forums, blogs, 3rd-party developers and free and commercial training resources. It also comes with additional utilities (Bridge, Camera Raw, the Extendscript toolkit for javascript, Pixel Bender, etc.) that enhance and speed up your workflow.
The sheer quantity of available tutorial material out there, whether video or written, is amazing. The free tv.adobe.com site is packed with video tutorials on individual products and ways to leverage the integration provided by different suites. Lynda.com has everything, of course, but honestly the resources are endless and most of them are free.
Dreamweaver's code editor, if you go that route, is strong, going well beyond just HTML and CSS. (Some love it, some don't -- it's a matter of taste as much as anything else.) Having a built-in webkit engine, easy AMP stack integration, automated round-tripping between DW and PS/FW for image updates, and automatic housekeeping, automatic relinking, etc., takes care of a lot of routine site management chores. The multi-screen capabilities in DW CS5.5 help when building sites targeted at mobile devices, and Device Central gives you a boatload of mobile device mockups for testing ideas.
The initial investment in a Web Standard or Web Premium suite is substantial, but it IS an investment, not just an expense (although it does come off your earnings as a business expense for tax purposes). Once you have a suite the upgrades are quite inexpensive.
Adobe's focus is always on speed of production in a commercial workflow, and my experience over the years has been that a typical upgrade pays for itself in saved production time within a very few weeks.