Wireless card differences?

I'm planning on purchasing a new laptop, but I really don't know anything about wireless cards. What exactly is the difference between my four options:

1. Dell Wireless 1490 802.11a/g Mini Card

2. Intel® 3945 802.11a/g Mini-card

3. Dell Wireless 1505 Wireless-N Mini-card

4. Intel Next-Gen Wireless-N Mini-card

I don't know what my wireless network is, but as far as I know it's fairly basic. I assume the first two are essentially the same thing, just made by different companies, but I have no idea what the benefit of a Wireless-N card is. Dell used to have a button to click to compare the different options, I remember seeing it a couple of weeks ago (although unfortunately didn't click it then), but that option seems to have disappeared for some reason. Would I even be able to make use of a Wireless-N card? Is there any reason at all to get one, or would it only be beneficial if I was on a different kind of network? I want to put in my order as soon as possible (preferably tonight), but I don't want to order the wrong card. If it's an upgrade (either with speed or distance that it picks up the signal or whatever) that I would notice on my regular home network I'd get the better card, but if it's only for a specific type of network I don't have access to, then I won't bother.

Thanks in advance, I hope to get a response quickly so I can place the order.