Building a new kernel was originally pretty much a required step in a FreeBSD installation, but more recent releases have benefited from the introduction of a much friendlier kernel configuration tool. When at the FreeBSD boot prompt (boot:), use the "-c" flag and you will be dropped into a visual configuration screen which allows you to configure the kernel's settings for most common ISA cards.
It's still recommended that you eventually build a new kernel containing just the drivers that you need, just to save a bit of RAM, but it's no longer a strict requirement for most systems.