SATA 1.5Gbps will only bottleneck an SSD in sequential speeds. The only sequential transfers are when you're transferring single large files. Please look at random R/W speeds, since thats how your disk is used the majority of the time.
Run CrystalDiskMark on your existing hard drive and look at the benchmark that delivers <1MB/s speeds - that is what an SSD is supposed to improve on.
- While comparing random speeds on SSDs, you might find some "cheaper" SSDs doing as badly as mechanical HDDs, effectively nulling the whole performance boost you should get with a good SSD. The performance difference between SSDs is huge and does not scale with price. Saving hundred bucks can drop performance by a hundred times as well, so be careful.
- As for the Intel 520, just one word - "Sandforce". I can't think of any notable consumer SSDs released this year using a Sandforce controller. If manufacturers are scared of touching it, why should you?
- Finally, if you're looking at resale value, then another point to consider is the limited P/E cycles NAND has. TLC NAND in the 840 Evo has about 1K P/E cycles vs 3~5k P/E cycles on MLC NAND. A TLC NAND drive can still have a lifespan of hundreds of TBs of writes, so you need to consider your usage.
