You don't have to buy one from Dell AT ALL.
Depending on the motherboard you have, it may limit the size of the hard drive that you can actually use. I had a Dell Dimension E510 with a Pentium 4 CPU a while back. The largest size the motherboard recognized was 320GB.
Check your manual for any note.Or you can let Dell scan your system here and see what type of hard drives show up as upgrade options. And go by that.
You need to look for the right interface type.
If your current laptop HDD is IDE / PATA HDD, you need that interface HDD. If your current HDD is SATA, then the Seagate will do.
Definitely the 2.5" type, not 3.5".