Actually, it was not available at all until just a few weeks ago. Garmin has now introduced the Northeast 24k topo's however - see here:
https://buy.garmin.com/shop/shop.do?cID=255&pID=68271#versionTab
AFAIK, these must be purchased directly from Garmin. It's possible that REI might carry them in local stores though, I haven't checked. The preloaded card should be usable in any Garmin GPS, but it is copy protected and cannot be duplicated so you must always use the original card (in other words, they can only be used on one GPS at a time). The card cannot be used on your computer in mapsource, only on the GPS.
The downloaded version will create a file on your computer which must then be installed on your own SD memory card. Once you put that card on your GPS, it will be locked forever to your 76csx and cannot be used on another GPS. So if you sell your 76csx, be sure to sell it along with this card. The download version also cannot be used on your computer with mapsource.
So actually, there is no version of the NE 24k Topo which can be used on your computer in mapsource. That is only possible with the version on DVD, and there is no such product with the Northeast 24k. Maybe they will introduce one later?
Also note that these maps are pretty expensive - $100 and they only cover a few states:
https://buy.garmin.com/shop/shop.do?cID=255&pID=68271#coverageTab. You can't buy one disk for the whole US, there are a bunch of different regional disks and it would end up costing you over $1,000 to get them all!
Let us know what you think if you get these. Since it's new, I have not seen any user reports and personally I don't want to spend $100 and take a chance. One nice thing about the 24k topo's is that the roads are all routable, unlike the 100k topo's which are not. This is why the 24k topo's are copy protected, because they contain Navteq data, unlike the 100k topo's which aren't copy protected.
The whole 24k topo series is very new - first introduced with a few western states about 2 years ago. They don't even have coverage for the whole US yet. I would not expect any updates in the forseeable future. And when they are updated, I am willing to bet that you would have to just buy the new version at full price, or maybe at a slight discount. I'm sure it won't be free though. Garmin has never offered a free update to any of their topo maps. It was perhaps 6 years between updates of the 100k topo maps for example.