Her old HP has a Crucial M4 I put in when it was new, but I think it was overkill for the CPU (1.65GHz AMD E-450) and the SATA 2 bus that model was saddled with.
You may tend not to believe this, but for normal users moving from SATA 2 to SATA 3 is a negligible performance win. The bulk of the performance boost we experience from SSD vs. HDD on normal machines (office, browsing, light games) comes from improved random 4KB reads/writes. Random IOPS sees no gain from the upgrade to SATA 3.
Another counter intuitive aspect is that low spec machines might actually
subjectively benefit more from HDD to SSD upgrade than high end machines. As you move down from a CPU performance and RAM amount perspective, SSDs cut delays by a higher percentage than they would do on high end machines, hence user will experience a more noticeable (relative) improvement.
AFAIK the only thing the Crucial m4 is missing when compared to "modern" drives is DevSleep support, a feature which enables lower power usage while system is mostly idle (in your case the drive would go from 0.6W in idle to something like 0.05W). However, if your wife uses the machine mostly at home, this feature can safely be ignored, as it brings you virtually no benefit.
Nowadays the rule of thumb when buying SSDs for light usage is very simple: buy the cheapest proven drive (with regards to both reliability and decent performance) that offers enough space for your needs in the following 2 years. Higher performance numbers are irrelevant for light users.
If your m4 is a proven reliable drive, don't chase a faster drive until you need more storage capacity.