Agreed with all the comments. SQL Server 2005 / 2008 are both very capable of handling that and more, the question really has to be "what do you have skills in supporting?" because ultimately there are areas which need more attention when we start looking at larger data sets. So, assess your company profile, technology direction and then make your decision.

If you don't have any deep skills in any of the major DB platforms then SQL Server will offer the easiest learning curve for the most part and will offer the lowest managability price point (an often neglected consideration).