Wireless Site Survey

Greetings all!

I'm learning the ropes on how to perform a wireless site survey.
I've looked around for what products are available (Ekahau,
Visiwave, LanPlanner, etc.).

Most of these product use an electronic floorplan to graphically
map out signal strength, noise and so on. Pretty neat.

However either many of these products won't support my Wifi card, or
I simply cannot afford the pricetag that goes with them.

I thought about using Netstumble which I found does work with my
hardware.

In order to reproduce the nice floorplan maps like the products
above, can I not simply mark down the signal results from netstumbler
and build a floorplan map that way?

Do I have to take certain things into account (echos from walls, etc.)
that may affect readings that product like Ekahau or Visiwave do already?

I'm open to suggestions.