Just to some up thing the way I see it:
1)I Love MySQL. It has served me well for years, but it's power is very limited and should be used only for relatively small applications (even VBulletin could be considered stretching it abit. Heavy traffic VBulletins like WHT suffer from time to time). BTW I recently did my company's review of MySQL 5 (since I'm an avid OpenSource developer my company often sends me to do the review of Open Source products) and let me tell you- its great. It handled Stored procedures and Inner Queries very well. There were afew screw ups, but it is still in Beta and works nicely.

2)For larger applications I would suggest Oracle (Not SQLServer), because it is much more powerful. Admitedly it can be overkill sometimes. Oracle server eat ALOT of memory, but if your looking for a well paying job your much better off learning Oracle rather than SQLServer, because it is muly platform, and has excelent support.

Good luck