Short answer: Yes. A good RAID0 implementation will notably improve your serial read rate, so going to one will reduce read speed somewhat. Programs will take longer to load, ....
Long answer: RAID 0 also triples (or so) your chance of system failure. Keep backups, including backups of the OS. Having only one HDD is my choice, for reliability. Of course, I blew the money on an SSD, and I have multiple external backups, because I am a G*E*E*K.
Edit: I just re-read this. You say going from RAID0 down to a single disk. Unless you have a lousy RAID0, I would expect the single disk to read noticeably slower, and write in the same range as the RAID0 or a little worse. Unless the one HDD is much faster than both of the ones you have in RAID now. RAID0's functions are increased speed and increased space, compared to a single drive.
i will only have the OS and my applications on this raid0, so reliability is not really an issue for me. and the hardrives in question are all the same. now I know that theoretically the raid0 is faster than the single hardrive, but we're talking about a normal desktop computer where I use it for internet, downloading, converting video files, and soon a couple of games. this is not a server. would I still notice a difference?