You don't say what type of PC or operating system you are running, but I'm assuming that it is fairly old and running Win95 or Win98 which still uses MS-DOS. If your current onboard BIOS is not compatible, you can still put in the EIDE drive, but you risk the old BIOS not recognizing the increased capacity, or giving you improper operation.
You will want an on-card Enhanced BIOS too if your mainboard BIOS doesn't support translation or if its support is buggy or outdated. The interface BIOS will override the mainboard BIOS' harddisk routines.
The Enhanced BIOS
A rather different issue was the 504MB (or, equivalently, 528 million bytes) capacity limit that becomes apparent when accessing IDE drives through the BIOS. It is caused by the disk geometry (cylinders, heads, sectors) supported by the combination of an IDE drive and the BIOS' software interface ('int13'). Both IDE/ATA and the BIOS are capable of supporting huge disks, but their combined limitations conspire to restrict the useful capacity of the drive to 504MB. Since only MSDOS still uses the BIOS for harddisk access, this is sometimes erroneously thought to be an MSDOS limitation; other operating systems experience the same restrictions at boot time though.
An Enhanced BIOS works around this problem by representing the drive to the software using a different geometry than the native geometry of the drive itself. This juggling act is called 'translation'. For example, if your drive has 1500 cylinders and 16 heads, a translating BIOS will make software programs think that the drive has 750 cylinders and 32 heads.
If you're not sure what type BIOS you have and whether or not it will support translation for EIDE HDD's, read Section 4.2 of this article which may help you identify what you have.
Section 4.9 of this same article will tell you the options you may have to get enhanced BIOS. All in all, this is a very informative article and will probably answer all your questions.