The specifications on a HPcompaq nc6400 are the following. Intel Centrino Duo processor technology, Intel Core 2 duo processor T5600, 1.83GHz, 2MB L2 cache, 667MHz FSB, Intel Wireless LAN 802.11a/b/g MOW1 mini-PCM-card and bluetooth.
The HP Compaq NC6400 can be bought on many online websites. Websites like eBay, Amazon, and Overstock has the HP Compaq NC6400 available in stock for one to purchase.
There are many possible causes of the black screen of death in an HP Compaq NC6400. One likely cause of this issue would include a failed display driver.
Since the only difference in your detail - is the amount of memory (4GB against 2GB) - go for the 4GB machine. The extra memory will help Windows 7 run faster.
As the specified computer, an nc6220 laptop is somewhat old, it can mostly be found online. As such, one can purchase it off of Amazon or Ebay and the like.
HP/Compaq nc6400 has 2 DDR2 (speed up to PC2-5300 (667MHz)) slots and can have maximum total RAM of 4GB. Memory module upgrades are farely easy and straight forward. Just get a compatible memory and add/upgrade in the memory slot which is mostly accessible from bottom of the laptop by removing one or two screws. Here is a tutorial for memory replacement/upgrade from HP for this series of laptops. http://bizsupport.austin.hp.com/bc/docs/support/SupportManual/c00713949/c00713949.pdf - Neeraj Sharma
on a laptop look on the bottom of it for "product" and get the info (brand and model number). go onto the website: opendrivers.com and type in the search box you computer type and model and "authentec" (compaq nx6320 authentec) to search for the computers fingerprint drive. click on your computers drive then type in the code it tells you. download the one that says "official server", DON'T download the one that says "automatically download and update all outdated drivers" because it downloads other software too. at the start of the installation it will tell you what other software you will need to download to get it working. download and install them by searching on google for them and then you will be able to use it.
On an outdated 7 year old laptop running Windows XP, you will be lucky to get $125. Take $100 if someone offers it.
One is on the bottom of the laptop directly in the middle secured by one screw and RAM clips. The other is below the keyboard which is secured by three screws on the bottom of the laptop, clips holding the keyboard, then the usual RAM chip clips on either side.