SSD on an Older Motherboard Worth The Investment? (ASUS P5KPL-CM)

Gonna cut and post from another answer I gave since this is the same question. Your motherboard has a SATA 2 interface (3 Gbps).

A HDD's sequential speeds are about 125 MB/s. A SSD's sequential speeds on SATA 3 are about 500 MB/s. 4x faster.

A HDD's 4k speeds are about 0.7 MB/s. A SSD's 4k speeds are about 30-70 MB/s. 40x to 100x faster. If the SATA interface supports NCQ (queuing multiple small file requests, instead of waiting for one task to complete before giving it another task), its 4k speeds can go up to 200-400 MB/s. So now it's several hundred times faster than a HDD.

So it's the 4k speeds which make a SSD feel so much faster than a HDD. It's not the sequential speeds that users and drive manufacturers obsess over.

You'll notice that these 4k speeds are almost all below the SATA 2 speed limit (300 MB/s). Only large sequential file read/writes will be slower with SATA 2. That's mostly benchmarks and (if you had a second SSD) copying movies from one SSD to another. About the only real-life task where high sequential speeds really matter with a single SSD is real-time video editing. The vast majority of files a computer accesses on a drive are small (4k) to medium (a few megabytes), and with current SSDs these will almost always fall below the SATA 2 speed limit.

So yeah, upgrading a SATA 2 motherboard with a SSD is worth it. The main thing you should be worried about is whether or not the SATA interface supports NCQ (since that can make about a 5x difference in speed of 4k read/writes). Some SATA 2 interfaces have it, some don't. NCQ is a part of the AHCI command set. So if the motherboard has AHCI mode available in the BIOS, then it will support NCQ and you are good to go. (This is also why it's important to change the computer from IDE to AHCI mode when you upgrade to a SSD.)

And to answer your followup questions, just install it like a HDD. The motherboard will see it and treat it like a HDD. It'll just be really fast. You can buy a bracket if you like. But SSDs are so lightweight you can just make a loop of duct tape and use it to stick the SSD to the side of the case or in an empty drive bay.