D-Link DIR-645 or Netgear DGND3700?

Can't comment on either but every D-Link piece of hardware I've bought has been a dog. Netgear is not too flash either but still ahead of a D-Link.
 
Choose the Netgear because it has dual band support. You will only get a maximum of 130Mbit/s via wifi if you are stuck using 2.4GHz on 802.11n. The DIR-645 doesn't even have gigabit ethernet ports so good luck getting more than 100Mbps between your wired and wireless net :)
 
One thing I must mention is an all in one solution like the DGND3700 will never be as good as a standalone product. I personally have a cheapo $30 TPLink ADSL modem that only does 1 thing - but it does the ADSL thing well. I can swap WiFi access point whenever I feel like without having to worry about ADSL issues.

I didn't give you the entire gist of the story. The TPLink modem is only working in bridged mode. I use an old PC as an Internet gateway and that handles IPv6.
 
In the past I have had a similar configuration using a wireless router without built-in ADSL (Linksys WRT54G), hooked up to a modem in bridged mode. It worked well. What I'm hinting towards is that you'll find there is much more choice for a good performance unit if it does not contain built in ADSL, such as Apple Airport.

Sounds like a plan. I'm just warning you that you won't get the best performance using 802.11n on 2.4GHz. This is because to get maximum performance, the radio must be able to use 40MHz channels. The problem is, 40MHz takes up almost 80% of the entire 2.4GHz wireless band. You will get lots of interference for sure. 40MHz channels will work much better on the 5 GHz band.

Make sure you buy a unit where the output power can be adjusted and antennas can be upgraded - make sure it has external antenna connectors. High gain omni-directional antennas can be had quite cheaply and do well to increase range.

I don't like the sound of that so called "SmartBeam" technology. It just sounds like marketing terminology to me. Only 4 antennas are supported under 802.11n standard. This allows for 4 data streams, yielding a maximum of 600 Mbps using 40 MHz channels. 
 
It is likely your laptop only supports 2 or 3 antennas anyway, meaning you will get 300 or 450Mbps in the best conditions.