My 5 year old laptop is running out of HDD. It's also getting more sluggish despite my attempts to clean it up. By the looks of things, the RAM is maxed out at 8GB (chassis based on Clevo P170HM).

I'm looking at SSD upgrades but am I correct in thinking there's no speed benefit above SATA I (as opposed to II or III)?

I have the original 500GB WD Scorpio HDD and I later added a Seagate 750GB Hybrid drive.

Any SSD will be able to saturate the SATA I interface. As you're already using a hybrid drive I'm not sure what exact benefit you'll see in normal usage as the hybrid will have cached a lot of your regularly used stuff in its flash section although I'd still expect to see some increase in performance with a SSD over the hybrid drive. It should be able to max the interface almost regardless of what's you're doing.

You might want to consider the cost though. If you're reaching the limits of the hybrid drive you'd need to step up to a 1TB or larger SSD. You're looking at the wrong side of £200 for that capacity point or over £500 for 2TB. Given the age of your laptop you might be better putting that cash towards a newer laptop (with a 1TB drive already installed and a faster drive interface as well).

Thanks. I figured I'd max out the interface with any new SATA drive. I was thinking I'd swap the boot drive for SSD (even if it's 500GB) and maybe swap the 750GB SSHD for a larger capacity one. Alternatively I need to find my Synology NAS, add some discs and transfer everything across. The things that take up most space ATM are either films/tv shows I've ripped/downloaded, or photos (I always keep the RAW files).

You'll notice a massive difference changing the boot to SSD.
Get a well known brand & you'll feel like it's a new machine.

The best hard drive and SSD deals in August 2016 | TechRadar
http://www.techradar.com/news/computing ... 14-1242806

An SSD makes the world of difference in day to day usage. I absolutely hate it now when someone hands me a more standard laptop.

I put a Samsung 850 EVO 500GB in my 2010 Sony Vaio and it made a huge improvement. Boot times went from over 30 seconds to under 15, in fact it booted as fast as my Surface Pro 3.

I thibk it was Samsung Evo SSDs that have been recommended in the past and was one that I had my eye on.