Some questions to ask yourself first.
1. Do I really
need Photoshop?
For most people, the full version of Photoshop is overkill. Many would struggle to understand, let alone use all of the features that the software is capable of.
2. If I don't need Photoshop, what else is there?
First and foremost there's Lightroom. All the RAW processing functionality from Photoshop but presented in a more logical manner and designed specifically for photographers. And it even does JPEG too.
3. What if I don't like the sound of Lightroom?
Then there's always Photoshop Elements, Adobe's cut-down version of Photoshop, aimed specifically at the photography market. It's got most of things you do need and hardly any of the things you don't. And as a result it's a lot more affordable than the full version of Photoshop and probably more suitable than Lightroom.
4. What if I don't like the idea of Elements?
Then there's a number of alternatives available to you, most of which are covered in
here. Some are free, some aren't. All will do what you need them to do, with varying degrees of capability and ease of use.
5. What if I still want Photoshop?
Then you're going to have to get the plastic out. There's various
discounts for the full version of CS4 doing the rounds, but even then it's still a hefty investment.