This is only half of the equation.
What you are looking for is an antenna with between 70 and 80 percent on the elevation plane. This would put you looking for a "patch" or also known as a "directional" antenna. Scroll down to almost half of the page, and look at this guy: AIR-ANT2460P-R
A patch antenna will work just fine in your house even with you moving around with a laptop. You will also find the other types of antennas, and specifics about each on this page.
Now to the other half of the equation. Wireless communication is half duplex and bi-directional. If you put a stronger antenna on your access point that will push a -75 dBm signal out 400 feet, the client will also need to be able to get a signal back to the access point. This is the reason when you design a wireless network, you match the dBm of the access point to the lowest powered client. This way you never leave a client out there that can't get back to the access point.
It is going to be difficult for you to verify you have enough signal from your laptop back to your access point without an enterprize grade access point. Yes DDWRT does have a signal meter in it, but if I remember correctly it is in RSSI. RSSI differs for each Wifi NIC vendor, and there is not standard for it yet. The only other option you have is an enterprise grade tool that shows client signal strengths.
Just make sure you add up the total power output for the access point which includes antenna gain, and compare that to the NIC in the client. If you need help getting total AP output power, just let me know.