if you have constant hard-disk accessing when on Vista, then you have far too many resources being used because it means Vista has had to fall-back to the Virtual Memory.
High-Demand applications like: AOL, Limewire, eMule, Kazaa, 4oD, McAffee, Norton, etc..
Kill Vista performance; seriously so!
Applications I'd recommend, that don't kill the performance even on a computer using 256MB Physical Memory.
Anti-Virus: Microsoft Live! OneCare
Firewall: Microsoft Live! OneCare
Spyware: Microsoft Live! OneCare
P2P: Bit Torrent (from Bittorrent.com)
Media Player: iTunes with Quicktime
Media Player 11, works alright but only once you've optimised the system or are running 512MB Physical Memory (preferably DDR2)
To optimise the system, I'd HIGHLY recommend SpeedCache to be turned on, and Indexing turned off. With Indexing Off, searching is slower.. but not even close to as slow as what it is on XP.. this said it also slows down your system because it constantly has to update the Index file.
I would also recommend changing your virtual memory setup to 384MB - 768MB (per Hard Disk) on ATA, 768MB - 2048MB (per Hard Disk) on SATA. If you have Raid turned on then keep to what ATA use.
The reason you want different sizes is because of bandwidth. More than 768MB on ATA and it'll just end up hurting performance.. Vista is defaulted for SATA which is why performance can seem poor.
I'd also strongly recommend ONLY using Windows Live Messenger, because AIM, Yahoo! and Trillian while not taking up much processor resources it takes up very valuable Physical Memory.
Oh as far as the VM goes, you could use down to 128MB - 512MB; but you should generally use 768MB/2048MB as the top amount because otherwise it's too large for bandwidth of the connection. Also you want more base RAM to begin with, because otherwise it takes longer for it to index the rest as and when required.
Less than 128MB and Vista can't initialise Win32 properly.
For more optimisation when you're an admin you can use the task manager to see what services are now running. Use this to make sure you have the basics running taking up the least amount of memory. Ignore processor requirements as unless you have the basic 800MHz CPU this just won't be an issue ever. Once you have a well optimised services list then switch to the services configurer, and set everything you don't need to "manual", those which aren't important for start-up (i.e. Microsoft Live! OneCare) set to automatic(delayed).
This will help optimise boot performance and overall performance.
Also I'd strongly recommend you set-up admin priviledges for each application you install, and also sort them between applications and games. It won't help performance but stop the annoying "accept" on every damn operation.