I use a PDA with an external GPS-antenna (Bluetooth) for navigation and have used it in Europe and in the States. I simply download the charts I need from the DVD onto my PDA's memorycard and then use them. USA (Incl. Alaska and Hawaii) and Canada requires about 1GB. The coverage 'Southeast', which consists of Florida, Georgia, Alabama, Lousiana, and South Carolina requires about 130MB. The software I use is Navigon Mobile Navigator 5.0.
A single USA-coverage is $99, the complete one (See above) is $279. If you have the DVDs, which usually come with the system, you can obtain codes to download additional coverages online at aforementioend prices - or if you really know your way around on the web, there are of course other possibilities to get those codes
One tip: When you fly to the USA and switch on your GPS the first time (And also when you return to Ireland) the system will need considerably more time to find satellites than usual. It's a matter of adapting to another geographical region, but after the first time it is back to normal. So don't reboot it after a couple of minutes, it may need up to 15 minutes to adjust itself.
As stated in my post I use Navigon Mobile Navigator 5.0 and can recommend it. You can also use TomTom. A friend of mine has it and says only good things about it, he used it in Europe and in Colorado.