At the moment, program compatibility is iffy - If it was coded during Windows XP's life, it will probably work okay; Anything before that will probably have 'issues', depending on complexity. (Blame this on Microsoft constantly 'tweaking' old Windows API calls instead of making new calls)
For good games performance, it looks like you will NEED 2GB of RAM.
Memory Management does not seem any better - On systems with 512MB, Vista thrashes a lot more than XP still. At 1GB, both touch the HD a lot less; I'd recommend 1GB as the absolute bare minimum RAM if you want to use Vista and run more than one app at a time.
Aero is a massive resource hog - It uses up massive amounts of system and videocard processor time and RAM. You don't want to have lots of Windows open with Aero (I routinely browse with 50-60 Opera windows open; You can't do this in Win98 because it runs out of resources/handles. You can do this in Linux, 2k, XP and Vista. You can't do this in Vista Aero. Curiously it seems to make Vista run out of resource handles in a similar Win98 does?!?!?!

)
You will need a 512MB Graphics card. A 1GB one would be better, when they come into existance.
256MB will be slow but usable.
128MB will be painful.
Disabling Aero speeds things up considerably - It does not disable hardware acceleration, just 3D acceleration. 2D window acceleration functions as it did before. You do loose a lot of the cool-guy window effects obviously.
One cool thing about Aero is that you can play (multiple) accelerated videos and they'll show through any transparency properly instead of being a big purple/green/blue/black rectangle.
Disk Cache is good - Linux like. Unused RAM is turned into disk-cache, apparently similar to how Linux does it.
Unfortunately, it seems to be reluctant to actually let go of the cache once it is made; There seems to be some minimum cache it will try to maintain that varies by some as-yet unknown factors (System load maybe?)
We think this is why we have problems with some high-end games - With FEAR and 1GB it seems to try to cache the large texture swaps that go on, but then runs out of main RAM for the actual game and ends up hitting the swap-file, resulting in nasty slowdowns during 'high-action' scenes.
2GBs of RAM seems to solve this - We can run everything at max without negligible difference between Vista and XP.
Disclaimer: Most of this is hypothisis based on observations; At the moment Vista is looking to be on par as an OS update, but with a metric arse-ton of eyecandy.
Purists like me will hate the new interface (I don't mean Aero effects etc, I mean the UI) - It tries to help, like XPs, and gets in the way, like XP's. Classic Control Panel forevaaar!