I have a 300mbps router with two antennae (tp-link td-wd8968). I'm building a PC and looking for a wireless network card. I have seen seen cards with a single antenna (150mbps) and cards with two antennae (300mbps); I understand the latter is for 802.11n which has higher data rates, but does it have any affect on the signal range?
Not really. Wi-fi cards that support using two antennas generally have an internal switch that will select one antenna or the other. The card will select whichever antenna has a better signal. This is called diversity, and it is used to counter issues with channel fading. Newer wifi cards can use both antennas at the same time. This is called MIMO and under certain circumstances it can allow a 2x improvement in bandwidth. In terms of range, diversity will allow the card to counter some forms of fading and can improve reception and reliability, but it will not increase the range unless you use a higher gain (more directional) antenna.
Having two antennas that are specifically sited as an array can make the pair of antennas more directional and this inevitably leads to higher gain. However, more likely is that the two antennas are not interactive and are used to achieve diversity
This whitepaper has some nice graphs. Have a look at page 7, where the radiation pattern of a dipole is shown. This is the kind of radiation pattern you have with one antenna:
You can see that radiation is high in the x and y direction, but poor in the z direction. That means, if your router is in the z direction, you're going to have poor reception. To improve your reception you can position the antenna differently. You can do that on both sides: on your computer, and on your router.
Adding a second antenna in a 90 degrees corner with the first one would mean that you're going to improve reception in, say, z and x axis. That means you have fair reception in the z and y direction, and good reception in the x axis.
However, the card will normally not use both antennas at the same time, but rather select the one with the best reception, so you will have equal reception in all directions.
Newer cards will use both antennas at the same time. In that case you want to position the antennas in one line, so that you're improving both x and y, but not z. Then, again, make sure that your router is in the x or y direction, but not in z.
Having two antennas is therefore especially useful for moving transceivers (laptops, phones, etc.) or for transceivers that need to talk to many others at the same time (the router). Your router has two antennas, so make sure to position them optimally for your setup.