SAN vs NAS-Secure Data Storage for Small Businesses?

DeePCedure asks: "I'm the network administrator for a small e-commerce solution provider that is growing very quickly. We need a host independant means to centralize our data storage, but we have multiple platforms including Linux, Win32, two generations of Mac's, BeOS, and even OS/2. A Storage Area Network(SAN) seems like the best solution, but most of the information I've found on SANs is geared toward large enterprises. Also, Fiber Channel technology being as expensive as it is, a SAN seems out of reach. Network Attached Storage(NAS) is probably the next best thing but NAS has it's own problems. Most of the devices I've found don't support all of our platforms, and the ones that do don't have very good security. How can I centralize ~100GB of data on a multiple platform network within budget? Also, how can I do it securely, with controlled access all the way down to the bottom level of the data structure? "