My D620 is almost a year old. It did not ship with the 5505 installed, I had to do a post sales install. (SIM slot is however present under the battery).
The 5505 is available from Dell SA - I chose to install this myself as the rates were a bit steep for 5mins of work by a Dell techie!
Simple procedure - Is available from Dell's website, (forgot the URL

) - USE THE WHITE/GREY CABLE !! (not the black one)
Use the Dell Mobile Broadband Card Utility (5505) as the driver for this card. At the time of post, the file R129300.EXE is what you need from Dell.
The software claims to be 'Vodaphone' ignore this...
As long as you have a data activated SIM with airtime (or more cheaply some kind of data bundle), you are 'a for away' - a new SIM will take several hours to be data activated
I have a Prepaid MTN SIM & a Prepaid Vodacom SIM, both work well - bit of a pain to swap them around, the notebook must be off or it's little silicone brain gets confused
Coverage / Speeds. I battle at work to get any Vodacom signal (even plain old GSM), at home, it's a bit better - I can get GPRS. If I leave the building @ work, I get Vodacom 3G. MTN for me is better. EDGE @ work, full 3G at home.
Hope this info is useful - let me know if you need more info, all of this is fresh in my head!