If the HDD in your old Dell lasted this long, it was probably of good quality. But HDDs have a finite life span, so most of its life is probably behind it now. Always make a disk image often of your system. That way if the drive crashes or otherwise dies, you can restore that image to the new drive w/o having to start over with everything.
A SSD also has a finite life span. The flash memory can only be written to a certain number of times. Again, back up. But SSDs are dropping in price and make basic everyday activity much faster. Unfortunately, your old Dell's MB probably still uses SATA2, which isn't as fast as the newer SATA III. But it will still make a world of difference with access time.
But don't get the idea that a SSD will speed up your gaming frame rates. It will only speed up the time it takes to load the game initially and any other occasional access the game may have to the drive.
Btw, no one uses 5400RPM drives for desktops anymore. They are mostly for laptops. Always opt for the faster 7200RPM drives; the industry norm. 10,000RPM drives are also available.