Keyboard and Mouse
Keyboard
It’s really quite simple. Natural keyboards suck for RTS. I have witnessed many top of the line FPS players, and they prefer flat keyboards as well even though they can re-bind their keys to suit their needs. However, it’s even worse for RTS because hotkeys usually cannot be bound and you have to reach across the keyboard (for Protoss, you frequently have to reach between ‘P’ and ‘Z’) for some important hotkeys.
The cheapest good keyboard I have come across is a generic Key Tronic keyboard which costs around $20. In recent times, I have grown accustomed to the Logitech Internet Keyboard(retailed at $22), which I feel to be an excellent keyboard due to the versatilities of its settings and wide range of adjustability for the user. However, I doubt that the difference between the products will affect your gaming performance. It pretty much comes down to using a keyboard that is flat rather than the new ergonomic style.
Mouse
I’ve been using the same mouse for 5 years with small breaks in between. I’d have to say that it’s the most reliable mouse that I’ve come across. It's the Logitech “bear paw” Mouseman(retail $20) which will be replaced by the upcoming Logitech Wingman mouse (which is the same design except that it scrolls and clicks a little bit better and is retailed at $40). In between, I used the Microsoft mouse (retailed at $20) with the roller. Both mice are extremely good precision-wise. I have heard that the Microsoft mouse is much better for FPS, but aside from the Logitech one lasting longer for me in the past, but both are good choices for RTS. It comes down to what hand position you use and which you are more comfortable with.
Kenn has pestered me about whether the PS/2 rate utility affects game play. I couldn’t answer at the time since I was in the middle of PGL, and I knew that if I changed my settings that I might not be able to play at my best. I have finally tried it, and it actually does help a great deal for me. I was under the false impression that the PS/2 rate would affect the mouse scroll, but I was later informed that it only makes it look smoother on the monitor. The differences were pretty immediate as I was able to aim on smaller units much easier.