I have some moral issues with an OS dictating a hardware purchase, especially when the previous [downgraded] version ran the hardware, there's just no excuse for this, I did however use old fashioned instinct and opened up the Synaptic package mangler [pun intended] and typed in Dell. Lo and behold it's a chip-set problem!
After installing the montage of dell system drivers and especially the chipset update whammo!! The wifi card works like a champ. I didn't know Dell had apt-get goodies in our repos but then again something told me to look.
Dell Latitude D620, wn3945ABG wifi card.
Pop out the keyboard and look under it [upper right] if you're in doubt of yours.
Now if your wifi is a different flavor , Broadcom, Lynksys etc... This still might help, Chipset is important for integer and DMA addressing of hardware.