While it's true that Vista/7 use a lot of RAM with prefetch, it actually makes commonly used programs faster, but doesn't slow down anything else since it will immediately release that memory when needed. However, the memory use for prefetch, isn't shown in Task Manager.
The Windows 7 performance monitor will let you see how much is being used for prefetch. Open task manager (CTRL + SHIFT + ESC), click the Performance tab, click Resource Monitor, and then click the "Memory" tab. The grey section is reserved by the BIOS, green is actively in use by Vista itself and other programs, dark blue is prefetch, and light blue is unused. The green section is what is shown by side bar monitoring applications and task manager in both 7 and Vista. XP's task manger only shows page file usage. The used memory shown in Vista is actively being used by not only the programs you can see, but also the ones you cannot, IE - viruses and spyware, anti-virus, anti-spyware, all the shit that comes with any apple program, steam, that bullshit Rockstar Social Club thing, AIM/Yahoo Messenger/MySpaceMessenger/MSN, all the bullshit installed by Dell, and so on.
While Vista does use more memory than XP, the difference isn't any different than the upgrade from 2000 Pro to XP. 2000 will run happily on 64MB of RAM, where as XP needs at least 256 to be barely usable. People forget so quickly. This was considered outrageous at the time, and XP was known as a resource pig until memory prices came down. Vista itself only uses about 700MB to 1GB for itself; everything else is other programs.
Just like 2000 and XP, it's all the shit people install on their PCs that runs in the background that actually uses up all the memory and slows everything down. Because of that little sidebar meeter and the hardware requirements, that were considered steep at release, Vista got a bum rap. As far as memory usage goes, it isn't really any different than 7. If you install a third party monitoring application in XP, you also find it isn't much different either.
Also, for those of you arguing over how much memory Firefox uses, the more you have, the more it can use. The rest goes to the page file. Right now I have 4GB of memory, and Opera is using 374MB. On my old work computer which had 512MB, Opera would bring the computer to it's knees if it got much past 40MB of memory usage.
Edit:
Here is a screen shot of my Vista machine with 2GB of RAM. Vista is using 639MB of RAM or about 1/3.
http://i35.photobucket.com/albums/d196/Mag-X/Desktop/vista_memory_usage.jpgHere is a screen shot of my Windows 7 machine with 4GB of RAM. 7 is using 854MB of RAM, or about 1/4.
http://i35.photobucket.com/albums/d196/Mag-X/Desktop/7_memory_usage.jpgThe only XP machine I have here only has 384MB of RAM, so it isn't really any good for comparison. My XP machine at work has 2GB, and it's usage isn't that far behind my Vista machine. For the record, my step-brothers computer has a Celeron and 1GB of RAM, and it ran just fine with both Vista SP1 and SP2.
Vista's only real problem is that it doesn't scale well below 1GB of RAM like XP and 7 (and 7 won't go that far below). Since you can get 2GB of RAM for $30 at this point, everyone would probably have forgotten about Vista's memory usage if it weren't for netbooks coming along. I just think it's so funny that people love to hate on Vista, while at the same time praising Windows 7, when in reality, they're almost identical.
