Wow...
Vista had a bad launch due to driver and software incompatabilities, necessary to rebuild aging archetecture of the OS. Within a few months, and especially by SP1, Vista certainly surpassed XP. Maybe not in brute speed, but in usability, due to a number of interface improvements.
As for being a memory hog, well, yes, it used more memory than XP. Then again, when XP came out, it was seen as a huge memory hog, and I certainly didn't have more than 512MB of RAM at the time (I'm not sure I even had that much). What Vista does is take advantage of systems with a lot of memory, loading frequently used applications before they are launched to speed up performance. Why even have a system with 2+GB of RAM if you just want it to sit there? Oh, and for whoever suggested XP-64, just...no. XP-64 came out too early, and has almost no support for applications and drivers. Vista 64, on the other hand, is very well supported.
As far as other things go, I recently saw a chart (Can't remember where, maybe Maximum PC) that represented OS security. Over 30% of XP installs had some sort of malware on them when tested, while Vista was around 5%, and with SP1 dropped to 3%.
Now that that's out of the way, on to Windows 7. I'll admit, I haven't booted into Vista in a month or so, but that's because 7 is a big step up. Anything that works in Vista will work in 7, because it's based on the same NT kernal (Vista was 6.0, 7 is actually 6.1), and it's faster than either Vista or XP at pretty much everything. There are some beta-esque bugs in the 7000 build, but MS already said they've tracked down and fixed over 2000 bugs due to the open beta.
I'm running 7 on my Gaming/general purpose PC, my small SSD based laptop (With an old Core Duo (not c2d) and 2GB ram), and my touchscreen kitchen PC (Yes, kitchen PC. Basically a netbook in a case. Atom, 2GB RAM, slow hard drive), and it is excellent on all of them, even giving me smooth eye candy on the netbook spec PC.
You can add me to the list of "Will buy 7 the second it's made available" people. It's going to be an expensive day, though upgrading those 3 and my Media Center...