You can keep it, your board has a PATA or IDE interface as well as SATA. But remember, the hard drive is the slowest part of your system relative to everything else. If you can budget it in somehow, an inexpensive SATA drive generally will be twice as fast as your old IDE, maybe even faster depending on exactly what the IDE drive is.
I would say that it is a necessity to upgrade from IDE to SATA. Serial ATA drives are so much faster than the old IDE drives and yet quite inexpensive.
I may be wrong, but I believe that most motherboards today only come equipped with one IDE channel, leaving you with only two IDE devices, since SATA is now the standard. You typically do not want to share a hard drive with an optical drive or any other drive on the same IDE channel. Suppose you have a DVD burner and your HDD on the same IDE channel - the HDD is the master and the DVD drive is the slave. Your hard drive's performance will be reduced because it has to share the IDE's bandwidth with the DVD drive.
Switching to SATA will provide your hard drive with its own bus separate from all others. You will see better load times with WoW when switching to the SATA interface, promise!
I disagree to some extent with aoster87. He / she is right that sharing the IDE channel between HDD and optical drive can slow down the HDD a bit. But that only happens if you are trying to use both devices at the same time, and how often does that happen? Well, if you're burning a DVD in the background while playing a game, yes. If that's you situation, re-think your plan or learn to live with that effect until you can afford to upgrade. But if you do that sort of thing rarely, you may never notice the effect.