I have a Mushkin Enhanced Chronos 120GB SSD (http://www.mushkin.com/Digital-Storage/SSDs/MKNSSDCR120GB.aspx). Its performance is rated as:

Capacity: 120GB

Read Speed: up to 550MB/sec

Write Speed: up to 515MB/sec

Controller: SF-2281

Interface Type: SATA 3.0 (6Gb/s)

IOPS: 90,000 (4K random write, 4K aligned)

My performance however is much lower.

Screenshots from HD Tune Pro 5.00:

I am however using this on a SATA2 port, while it is a SATA3 SSD.

The other hardware:

Motherboard: Gigabyte ga-ma790xt-ud4p (http://www.gigabyte.us/products/product-page.aspx?pid=3010#ov) (latest BIOS (F8))

Processor: AMD Phenom II x4 955 BE

RAM: 16GB (4x4) Corsair Vengeance (http://www.corsair.com/vengeance-8gb-dual-channel-ddr3-memory-kit-cmz8gx3m2a1600c9.html

Graphics card: ATI Radeon HD4890 1GB (http://www.amd.com/us/products/desktop/graphics/ati-radeon-hd-4000/hd-4890/pages/ati-radeon-hd-4890-overview.aspx

It has always been this slow, I haven't seen the performance go down since I started using it.

EDIT:

Here's a screenshot from ATTO. It looks a bit better then HD Tune Pro.

After enabling AHCI:

The first problem you have is that you are using HDTune for the benchmark. The way it does its benchmarking is to try and map out how the performance of a hard drive changes as you move across the platter. This type of access is a worst case for SSDs and does not lend itself to pushing an SSD as far as it can go. The rating for SSDs are usually done with ATTO Disk Benchmark. As a benchmark ATTO has the opposite problem. It is a best case scenario, which is why the ratings say "up to".

Also, being on SATAII will limit you to around 260-280MB/s.

Another common cause of not getting the expected speeds from a SSD is if the SATA controller is in IDE emulation mode instead of in AHCI mode.