[BarracudaUAK]

-- EDIT -- Forgot the whole reason for the info...

Short Version, yes it will help.
I started typing this, then thought I should put up some comparable figures to show the difference, and then it became this:
-- end edit --

Your SATA III are 6Gbps,

6 Gigabits per second, Which last time I checked for a MegaBYTE number on that, it was something around 300 MegaBytes per second.

Which is only the Cache speed... actual read/write speed will be slower.

Last numbers I saw on Western Digital Caviar Black 7200rpm (the performance drive) was about 165-167MB/second sustained read speed, with a SATA III.
I have SATA III "Seagate Desktop" 7200rpm (formerly "Seagate Barracuda") (nope, not where I got my name from), they have a sustained read speed of 150MB/Second.

In My previous PC with a 10k rpm 300GB WD Raptor: SATA II has a 3Gbps Transfer rate, but will average 310MegaByte/Second read speed. (in linux) (loads a crysis level in windows XP in 45 seconds.)

My current one with the Seagate desktops, will read 150MegaBytes per second, from each drive. But I have it in Raid0 (striped) and the read speed varies based on the partition.
Average: 718MB/S-559MB/s (minum 400MB/s at the 'end' of the drive. Max 765MB/s at the 'begining'.) With 4 HDDs.
When I only had 2 drives, it would sustain about 375MB/S average.

You get a bigger boost with more drives in raid, but you can "mimick" the effect with 2 individual drives almost as good.

On all of my windows gaming machines over the last 16/17 years, I kept OS, with swap file, on one, Games on the other.
UNLESS the newer drive was faster: Example: 13.6GB 5400rpm OS drive/ with 100GB 7200 RPM secondary drive, was faster to have swap on second drive. <--- this one was a ways back.

Barracuda