Jink
Assuming yours is the older Sierra 750 card and not the later Optima 3G card (both being Voda badged) you have an excellent product, tri-band so works in Europe and USA (I've used it widely).
The Voda charges are about £8/MB (you need Onspeed -
www.onspeed.com - in this case, seriously) and anything up to 2x that abroad.
GPRS works widely; 3G is very patchy.
There are two ways to use the card software-wise: the Voda software or the little program off the Sierra website which I think is much nicer. Buth give you a straight network connection which "just works".
You have to go to the Voda website and enable adult content, otherwise their "porn" filtering will get triggered when viewing random binary data like aviation weather and such
The best way to use GPRS is obviously to avoid massive transfers, so one runs one's life a bit differently. For emails, use a different email program which enables just header download and perhaps the first 100 lines, otherwise somebody sending you a 5MB playboy centrefold scan will use up your entire credit! Same with websites; many you just don't go to.
If you install Winfax 10.02 you can even send/receive faxes...