Just saw your edit sorry. Don't worry about using GarminExpress anyway, it's still too flaky at present imo even if you do decide to flick Win3.1 eventually ;).
[Only registered and activated users can see links. Click Here To Register...] [or GE] will automatically detect the current fw version when the unit's connected and download the latest which it will place in your unit's Garmin folder as GUPDATE.GCD. On the next boot, the unit will update itself. You can do the same without the Garmin software by simply renaming nuvi7x5_GCDFile__450.gcd and placing it there manually. Or you can drag nuvi7x5_GCDFile__450.gcd onto the WU icon on your desktop and it'll do it for you provided the unit's connected to the PC.
I personally dislike using gupdate.gcd, although it's the easiest way to update. I prefer to use
[Only registered and activated users can see links. Click Here To Register...] and
[Only registered and activated users can see links. Click Here To Register...] and an rgn firmware file [another story].
A bit of info about GE, it's a one-stop-shop software which [should] inter alia combine the purposes of WU [update of firmware and softs such as voices etc] and MapUpdater [which will update any maps to which the unit's entitled of course]. WU can't update maps, but in certain circumstances MU may prompt you to update fw before maps.