It will speed them up some, but not a whole lot. A 9800 would have a much bigger impact if you do a lot of 3D rendering or hardcore gaming, but for Photoshop, QuarkXPress, Final Cut Pro, etc. you'd be better off getting more RAM or a processor upgrade if you want to shell out the money.
I also recommend you double the amount of RAM you buy to 512 MB, minimum, depending on what your needs are. What do you do, anyway (Photoshop, video editing, etc.)?
• 2.5 GHz MacBook Pro Core 2 Duo, 4 GB RAM, 320 GB hard drive, runs 10.6.8
• 1.6 GHz iMac G5, 1.5 GB RAM, 120 GB hard drive, runs 10.4.11 (out of commission)
• iPhone 4, 16 GB, iOS 4.3.5