This is probably one of those questions for which there's no true correct answer, but I'm interested in peoples opinion.
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One big connection, say Gigabit, is ideal for your primary data backbone. You can still use other, smaller connections for other smaller networks.
One comment which failed to appear in all the responses thus far, is that of 'redundancy'. If you plan to setup a pair of servers in an H/A environment, you should seriously consider two network interfaces.
- Christoph