Leopard Server - Network Load Balancing

What do you mean by 'network load balancing'? That term means different things to different people.

If you mean binding multiple interfaces together to increase bandwidth/redundancy, then you cannot do that in your setup because it only works on multiple links to the same switch (i.e. you cannot bond a wireless and a wired ethernet port together, only multiple wired ports).

That likely wouldn't help much, anyway - unless your upstream connection is greater than your link speed (e.g. 1gpbs) then you're going to get throttled by your router anyway.

If that isn't your intent, then please explain what it is you're trying to achieve.

Well, my home internet connection is around two megabytes per second.

Do you mean two mega bytes per second, or two mega bits?

Network bandwidth is usually measured in bits, not bytes.

Even so, that's your connection speed between your router and your ISP. Having multiple connections to your router isn't going to help any - you could have 100 connections all running into your side of the router, but they're all going to get throttled by the bottleneck at the router - it isn't two megabits per connection, it's two megabits shared by all the connections.

So, given that, there is little point in trying to connect both links from a speed perspective. There could be some benefit from a network redundancy standpoint if you could do it, but you can't bind different interface types, so that's moot.
On the other hand, if you already have an ethernet cable running to your system anyway, you might as well just use that. It will be more reliable than your wireless connection and as I've already pointed out, you won't get any faster performance by using multiple links, so you're not losing anything.