Tomtom Gps Australia

GPS Questions.?

I am going to Hawaii in May from Australia.I found a Tomtom GPS at Walmart for $168.About half price compared to Australia.Seems to have good feedback and reviews.It is a Tomtom one third edition.What I would like to know is
1)If it has a USA map on it and I bring it home and down load an Australian map,will I loose the USA maps?Intend to go back to the USA in the future?
2)Would it have an internal memory or do they also use a SD card?
3)Can you get down loads from other map companies which would be cheaper than the Tomtom maps?
I ask this because Tomtom seem to want to charge as much as the Gps itself.
Hope you can help and thanks.

ive got a tomtom, they are good. Aparently they are the most accurate.

Garmin nüvi 205W 4.3-Inch Widescreen Portable GPS Navigator $179.95 GARMIN NUVI 205W PORTABLE GPS…

Garmin nuvi 200 3.5-Inch Portable GPS Navigator $159.00 Navigate without breaking your budget with nüvi 200. This affordable entry-level personal travel assistant comes with preloaded maps for the continental U.S., Hawaii, and Puerto Rico. For even more mapping options, nüvi 250 features preloaded maps for all of North America, while the transatlantic nüvi 270 includes preloaded maps for both continents. Like all nuvi 200-series members, the 200 fea…

Magellan Premium Car Kit for iPhone and iPod touch $79.59 AMPLIFIED HANDS-FREE CALLING USING BLUETOOTHIN-CAR CHARGINGPOWERFUL SENSITIVE BUILT-IN GPS RECEIVER PROVIDES FAST & ACCURATE DIRECTIONS EVERY TIMEWORKS WITH MOST NAVIGATION OR LOCATION-BASED APPSCRADLE IS COMPATIBLE WITH MOST GPS NAVIGATION APPS WHILE OPTIMIZED FOR THE MAGELLAN ROADMATE APP FOR IPHONEIN-CAR CHARGINGNOISE-CANCELLING HANDS-FREE SPEAKERPHONEPOWERFUL AMPLIFIED SPEAKER USE THE VOLUME C…

Sony XNV-660BT 6.1-Inch WVGA In-Dash A/V Receiver with Bluetooth, USB, Satellite/HD radio ready, and Integrated Navigation from TomTom $399.99 With the most accurate maps and over 1 million more miles of road in the US than others, plus features like USB 1-Wire connectivity and Bluetooth technology, you’re ready for the road trip of a lifetime and your daily commute with the Sony XNV660BT 6.1″ Touchscreen A/V Receiver with Navigation….

Sony XNV-770BT 7-Inch WVGA In-Dash A/V Receiver with Bluetooth, USB, Satellite/HD radio ready, and Integrated Navigation from TomTom $1,199.95 With the most accurate maps and over 1 million more miles of road in the US than others, plus features like USB 1-Wire connectivity and Bluetooth technology, you’re ready for the road trip of a lifetime and your daily commute with the Sony XNV660BT 7″ Touchscreen A/V Receiver with Navigation….