You can 4GB of memory into a vista machine.  System resources are usually allocated the first GB or so of memory so you will only see around 3 GB (whatever the remainder is after the system allocation) available under system resources.  I've run my Vista machine on both 2GB and now 4GB of RAM and to be honest have not seen a significant improvement.  I never get close to using it all and stay somewhere around the 1.5 GB range usually.  You can put 3GB in and have enough memory for Vista but if this is a dual core machine, I believe you will not get the expected speed out of an odd number of RAM modules.  It's my understanding that the odd RAM module would run at half the speed it would if it had the second stick.  I may be wrong on that so please check other sources.

A 4-slot computer can have 2 X 512MB plus 2 X 1 GB for a total of 3 GB, dual channel, provided each pair of modules are matched spec.  3 GB in Vista would roughly equal 2 GB in XP in my experience.  As you can see in your other post, there are some staunch defenders of Vista who have not experienced some of the issues you have.  A quick perusal of the Dell Vista and XP forums (not to mention countless internet forums and blogs) will confirm you are not alone with your concerns.

                "Vista is just slower"....LOFL

 

             My Vista pc's make XP archaic, I would NEVER go back to XP