GIMP will not achieve professional results. If you're hiring professional web developers for smaller projects, hire them based on their results, and buy the software they'll use. This will save you time and money, and save them frustration. Yes, it's expensive. But so are professional developers.
The industry standards are:
- Adobe Creative Suite 2
- site mockups
- photo touchups
- major design drafting
- Logo, brochure, other vector design
- A small minority of designers use this over Dreamweaver.
- Macromedia Studio 8
- site management
- Limited access content contributions to HTML
- embedded video
- interactive interfaces
- advanced animation (if required)
- Graphics (not picture) optimisation
- graphics touch-ups, etc.
Macromedia and Adobe recently merged, so you're now able to buy the whole lot as one package
Alternatively, if you're just getting started, just get Dreamweaver and Photoshop. They can cover about 80-90% of jobs happily. If you're getting into serious professional business, it's not fun to have to say no to 10-20% of the market, especially when interactive and vector works are paid the most.
Also, it's the convention not to bother upgrading to the next generation. Usually people skip one or two gens, because the improvements are slow & incremental, and upgrading is an unnecessarily costly business. Unless you're using intel based macs, don't upgrade to CS3.