Toms posts a monthly list of the best SSDs at each price range. I very highly recommend consulting that
Your motherboard lacking SATA III is not a huge deal. Any SSD on SATA II will still vastly outperform any platter drive on SATA III. You will not get the best possible performance from the SSD but it will still be a huge improvement
It is definitely worth it! and realistically it won't impact the performance of the drive much at all, my reccomendation for an inexpensive no frills drive would be the Crucial M4, not sure on the 128 gig ones, but in the last week I have seen the 256 gig ones as low as $180 with free shipping.
My current brand recommendations for SSD's are for Samsung 830 and any Intel.
They make their own nand chips, and have the werewithal to do better testing and validation.
120gb is a good size, expect to pay about $1 -$1.20 per gb.
The real value of a ssd is the random i/o response time which is minimal, 50x that of a hard drive. That is what the os does mostly. It will make desktop operations feel so much quicker.
Sequential speeds can be impacted by 3gb, but you will not notice it, except in synthetic benchmarks. It will still be 2x the speed of the best hard drive.
I think a ssd is absolutely one of the best performance upgrades you can make to a pc.
Is it worth it?
I think so.
But your circumstances may differ. Only YOU can decide worth.
Yeah, SSDs are definitely still worthwhile if you only have SATA II. My motherboard only has SATA II, and my Crucial M4 128GB is still quite fast in day to day use. As said above, SATA II will hold back an SSDs performance in benchmarking scenarios like sequential reads and writes, but in real world use, it's mostly 4k reads and writes that are done, not sequential, and 4k reads and writes are not affected by the slower speed of SATA II.
From a performance and longevity standpoint, nothing touches the SSD's with premium toshiba toggle mode flash (Patriot Wildfire, OWC Mercury Extreme Pro 6G (latest revision), OCZ Vertex 3 Max IOPS, and Mushkine Chronos Deluxe). Add proice to the ix and the Chronos Deluxe stands alone.
To sum it all up with a bow on top, [with the Chronos Deluxe] you get amazing performance, extremely long service life and a hassle free low price point on a drive that literally has very little competition in the marketplace.