The HDD is the Biggest bottleneck for system performance and SSD, even on sata II is a large increase in performance.
I would still go for a Sata III SSD, preferably the Curcial m4 or Samsung 830. The biggest performance boost when moving from a sata II port to a Sata III port is in the Sequencial read/writer performance. Two things here.
A).. This is the least important Parameter for an OS + program drive. Smal Random 4 K is much mor important and is not bottlenecked by sata II currently.
B).. Sata III SSDs that use Async NAND (lower end Drives - ie Agility III) and the Sandforce 22xx controller only show a big jump when using a Benchmark like ATTO. This is a HDD benchmark not no longer relavent for a OS + Program SSD, Reason is it uses Highly Compressable data, Not real world then this is coupled with the use of slower Async NAND chip.
The newer sata III SSDs using either Synchronous NAND or Toggle NAND will shou increased performance for the 4 K random performance on sata II over a sata II SSD.