OP, I have some practical questions for you. Firstly, how many employees do you have?
Secondly, how old (for the most part) are the ones who will be working with Access (and its replacement)?
The reason I ask is because if you have a fair number of employees (say more than 12) you might find yourself spending a lot of time teaching them how to use the new database software. If it's even moderately different from Access, count on some confusion, frustration, etc, in the beginning. Granted this will go away with time, but remember, it will be there for a while.
The reason for the second question is because older employees are more resistant to learning new software. I know it's stereotypical, blah, blah, but it's true. Keep this in mind.
Now, I don't recommend going with OpenOffice (or NeoOffice). Sure it's got the Access equivalent, but it's slower and if you're serious about running a business, you can't have that. iWork is nice, but really won't do it for you. I suggest keeping Office (the next version will be out in January-apparently

), so wait until then if you do decide to go Mac. There are plenty of good reasons to keep Office. Your employees know it, new employees will know it, it's fairly consistent when moving large files (yeah yeah iWork can export, but come on, let's be realistic about this), etc. Trust me, you'll want Office there.

In the end, I have to ask, why are you desiring to ditch PCs? I know I'm about to be flamed for this, but what if the reason you want Macs doesn't actually exist? Then it would have been a waste of time, energy, and money.