Yes, you can use a SATA 3 SSD on SATA 2 ports. Theoretically it will be a bit slower (than it would be on a SATA 3 port), but I'm not sure that you would notice the difference. In normal applications the big difference that an SSD makes is not so much the transfer time as the access time, which is limited in a mechanical hard disk by the time that it takes the heads to physically seek to the correct cylinder - and then they have to wait for the correct sector. With an SSD you have none of this overhead.
This should make a big difference to your Dell and is a very wise purchase now that the prices of SSDs are so reasonable.