About the concerns on memory:
This is true. While using 512 MB of RAM when idle may seem (and may be) rather silly, it won't be a big deal for very long. Most people who get Vista will get it on new PCs, and (hopefully) those PCs will be equipped to handle Vista well. Furthermore, although I haven't tested this as much as I would like since I have yet to load Vista on one of my primary machines, memory management has improved in Vista and hopefully virtual memory management will be reasonable and your computer won't crawl too much even if you approach the amount of RAM you have in memory usage. Anyway, I remember that about 10 years ago, I got a Windows 95 machine with 16 MB of RAM and it worked just fine, and now people don't mind that Windows XP hangs around 100 MB or so at boot.
As for the price, I dunno. I get a nice deal on some Microsoft software through my university (otherwise I would definately not be quick to upgrade). I really wish they would allow you cheap additional licenses if you bought one license (What if I want to load it on my desktop and laptop? Why do I have to shove out $400+?), or offer some "family pack" similar to Apple, where one copy costs $130 but $200 gets you five licenses.
A lot of the new Vista apps are going to be available for Windows XP as well. Windows Defender, IE7, Windows Media Player 11, and Windows Desktop Search (3) are all available in beta form for Windows XP. I'm not sure about DirectX 10, come to think of it I haven't heard anything, but I always kind of assumed there would be an XP release.
[Edit]
Mahjong Titans is a lot like an old Windows game called "Taipei," which came in some Entertainment Pack for Windows (3.1) that a lot of people had. Some of the other games included were SkiFree, Chip's Challenge, Tetris, and a few card games. Fun times...
[Edit 2]
For further reading, gotta love Wikipedia...
http://en.wikipedia.org/wiki/Features_new_to_Windows_Vista