Is there need for RAW conversion software when you have photoshop?

Basically if i want to buy the Nikon d5000, their RAW conversion software is sold separately. But i'm wondering, because i have photoshop, will that extra software really be of use and should i buy it? Because obv i DO wanna take raw photos

No if you have a later version of Photoshop it works with RAW photos.

You need the CAMERA RAW update file from Adobe… It is free, and you need to find the right version for your camera…

Photoshop RAW viewing and editing is handled by a small add in program, Microsoft image viewer calls this a CODEC, Adobe calls it a plug-in. Installing the RAW handling program from your camera maker will also install the plug-in in the Adobe folder titled "Plug-ins" automatically. Other poster is correct, newer versions of Photoshop already have plug-ins for most RAW formats but your camera is pretty new and Nikon won't stop changing their RAW format parameters so install Nikon View NX after you install Photoshop to ensure you have the most current plug-in.

As long as you have the latest version (cs5) and the latest update of their RAW plugin then no, you don't need anything else.

Yes, Adobe Photoshop still needs RAW conversion software (a plugin) to open RAW files. It's called Adobe Camera RAW - and it's free.

Adobe Camera RAW comes packaged with Photoshop. So long as you have an up-to-date version, you should be just fine. If your camera is very new you will require Photoshop CS5 and the latest update to Adobe Camera Raw (It's continually being updated to include the latest cameras on the market)
If you don't have the above then you will need to look into some other form of RAW converter to allow you to edit.