Having a MacBook gives a certain insight into this that others may not have.  I can say, with certainty, that at OS Loaded, Vista has allocated roughly 3 times as much RAM as Tiger 10.4.10 does.  At OSL (OS Loaded) + 30s, OS X is completely usable and applications can start opening, Vista is still doing stuff that prevents anything from opening.  At OSL+60s I have finished opening iTunes, Safari, Adium, Transmission, Coda, iWeb, Pages, Numbers, and Sketchup and have right at 1GB of 1.25GB RAM allocated in OS X.  Under Vista, at OSL + 60s, I have Firefox, Freecell, and Sketchup loaded and am using 1GB of 1.25GB of RAM.
I get that Vista has more background processes (file indexing, precaching, other you know OSy things) but I wish it was a little more responsive.  IMO, and from personal observation, Tiger is definitely better than XP in terms of memory management, especially in giving up allocated RAM.  You close an app in Tiger, that RAM is freed up instantly, not always the case in Windows.  I have noticed that Vista has done a much better job of it, but after having used Tiger for so long (and now Leopard Beta as well) I really have trouble handling Vista and its shortcomings in terms of responsiveness.
I do not care if Solitaire has a 8D interface now that requires DX8594 and a graphics card with 17TB of RAM... how does that let me do my work faster or better?