You should be fine. Some HDD's even have a jumper to switch the mode to make the drive run at SATA 1 speeds. Hopefully you'll be able to see it if its there on some screenshots. I've heard some drives have auto-detect now, so that's even better!
It'll be just as fast... you won't see any real difference in speed. Unless you have a massive RAID setup you're not telling us about. =)
Nobly is right, SATAII drives have jumpers which allow you to make them run at the slower bus speed. Move the jumper on your hard drive to the appropriate location as shown on the jumper configuation map on the hard drive itself. Once changed, you're all set.
This is a duplicate thread - next time look better.
In short, SATA-II drives (ie. 3 Gb/sec + Native Command Queueing) are backwards-compatible with SATA-I motherboards. Some can set up their mode using a jumper, others require an OEM tool (Hitachi) freely available on the Web, most are set up as SATA-I drives by default.