I'm reaching ~1250MB/s with 5 Intel SSDs in SATA/300; who needs SATA 6Gbps? ;-)
For HDDs, SATA 6Gbps is nothing more than marketing. For SSDs it allows for faster SSD controller designs to make use of the available bandwidth.
The Micron controller in the Crucial C300 is the first to have a 6Gbps controller, but doesn't really use the additional bandwidth. The only improvement is slightly higher sequential reads and 10% higher random IOps. Nothing ground-breaking. For that, you have to wait for the next generation of SSDs to arrive around christmas this year, when Intel will release its Postville refresh based on 25nm NAND MLC and 6Gbps controller. Other controller designs will likely introduce itself around that time.
So if you can wait, in half a year you'll have very fast SSDs. For now, a single Intel 40GB or 80GB is a nice buy and would still be usable in a second system after you bought a 'real' 300GB SATA6Gbps Intel SSD in january 2011 or so.