Your computer has an older PCI bus system, which has a maximum data transfer speed of 133 Mb/s, well suited to all IDE units. (Note, however, that the bus capacity is shared by all peripherals connected through it. That includes your IDE ports which ARE using the PCI bus, even though not via an add-on card in the slots.) That bus speed also can handle the typical data transfer rate of SATA mechanical (rotating disk) hard drives. HOWEVER, the whole point of SSD's and the new SATA 6.0 Gb/s interface system is that they can move data much faster than any previous mechanical drive system. But even if you had a SATA controller plugged into your PCI bus to connect an SSD, the PCI bus could not handle data any faster than a mechanical hard drive, and certainly not as fast as the SSD can run. So you would not see the benefit in speed.
Now, you don't say what kind of HDD you have already. It may be older and hence slower than today's HDD's. So you might get some speed boost by buying a current IDE drive to replace you older one. Before doing so, however, verify with Dell that your machine can handle HDD's over 137 GB in size. That is, in tech jargon, make sure your system has support for "48-bit LBA" on its IDE ports. Plain "large hard drives supported" or "28-bit LBA" is not good enough.