if you get an ssd with a good controler then ull see some real advantages no hdd reaches even 150MB\s , the ssd would more than likely reach its peak bandwith of 150 some ssds can even reach 280MB\s no hdd can even hope for that, untill next gen hard rectangular drives
How can a car be faster on a 150KM/h road, than walking on another road that has a speed limit 300KM/h? Answer to this question will also answer yours.
The truth is, that HDDs can't even get to 3MB/s when doing complicated I/O such as booting/application loading etc. HDD can only be fast if it reads "sequential" like when reading a large file. An SSD can be fast with complicated or "random" I/O. Thats why SSDs boot much faster than HDDs.
Modern SSDs like Intel X25-M and OCZ Vertex are SATA/300 though. And when reading you may get close to this.