any computer (mac, windows, whatever) is as secure as you make it - after a fresh install using the latest software, a windows box is pretty much good to go, as is a mac.
if you use risky software, don't run software updates, download things you shouldn't, etc, etc, then any box will be at risk. windows is a more likely target because of the majority of users. macs are built from unix, which is historically a more reliable system, although it has security problems as well from time to time.
so a mac is no better or worse than any computer at browsing to secure ssl websites like your bank site. it's what you do with the computer when you're not on those sites that will cause trouble.