I have a 2005 specialized hardrock sport which has an old fork I want to change out. I know the fork I have is a 1 1/8" threadless around 200mm. So I found this fork on nashbar that looks right, but I wantel to be sure before I buy it. Heres the fork. http://www.nashbar.com/bikes/Product...2_174928_-1___
That'll do fine. If you're keeping your existing headset, be prepared to have someone remove the crown race from your old fork and press it on your new fork.
Another option is to get a new cartridge bearing headset (which will need to be pressed into your frame's headtube by a shop with special tools) with a split crown race so you can change forks in the future yourself, if you ever decide to go back to suspension.
Im a little concerned about the geometry if I change it out for the rigid nashbar fork. I know the fork I'm replacing has 100mm of travel. Its a rst Gila t5. Is there anything other than the 1 1/8" threadless and the length it needs to go into the components by the handlebars?.