Re: Red beret dusted off ...
Going through in order:
Safari's fine, unless there's a particular Firefox plugin you need or you're mentally chained to Firefox. Safari can import Firefox bookmarks.
Mail is better than Thunderbird with respect to search and backup; Thunderbird stuffs your shee into one big file, whereas Mail has one file per message. Mail can import your Thunderbird mailbox.
I tend towards native Cocoa applications because they reap the benefits of things like text services (all Cocoa text entry boxes have on-the-fly spelling and grammar checking, for example) and because they just look nicer.
Thunderbird's erratic treatment of IMAP is also annoying; it doesn't seem to check for messages anywhere other than the inbox, which is quite frankly pants. That's underpants, not trousers. PANTS, I SAY!
I have a 24" iMac with a 256MB nVidia 7600GT; this quite happily runs at 1920x1200 in millions of colours.
Airport is wifi. It'll work or it won't just like any other wifi adapter. 70 metres is wildly optimistic, unless you have nothing other than air between the laptop and the AP.
Have a go with Safari, and install the Inquisitor add-on. Consider using Privoxy for ad-blocking (better, in my experience, than ABP on Firefox).
If you've got a stiffy for Gecko, consider Camino. It's a Gecko browser that has a proper Cocoa interface, unlike Firefox (which just fakes its Cocoa interface elements).
There are other browsers, like OmniWeb and Opera, but they add little to the browsing experience other than unfamiliar interfaces.
Picasa for OS X is cack; I'm pretty sure the same will be true of Chrome (and, let's face it, no web browser written by Google is going to have anything resembling a decent ad blocking solution). Google's applications for OS X aren't great, in the main. Google Earth is pretty good though.
I wouldn't bother installing Adobe Reader unless you have a PDF that's unmanageable in Preview.
You might want to investigate Adium for multi-protocol IM, GPG Keychain Access for managing any GPG bits you've got and Steermouse for controlling extra mouse buttons.
