i know xp doesnt use more than 2g of ram, does vista

The 64-bit versions of Vista can address from 8GB to 128GB+ depending on version. The 32 bit versions have the same limits as XP.

Not quite. Here's some reading for you, here and here.

i currently have 4gb of RAM on my Vista x64, runs fine without any BIOS changes

I have 4 gB of RAM on 2 Vista 32 bit systems now, and it's working just fine. Both systems only admit to 3.326 gB of RAM, but there are no issues with running programs or operating.

Ok let's make a correction of two

The gentleman speaking about the correct useable space is somewhat correct. XP has a limit of 2gb Virtual Address Space, and and 2gb kernel address space foe the purposes of use. You may change this to 3gb and 1gb using a switch in the boot loader.

this page is just a reference. MS if you read this, I nested deep in the site for accuracy.
http://msdn2.microsoft.com/en-us/library/aa366778.aspx

here is the KB
http://support.microsoft.com/kb/929605

Backing of this
http://support.microsoft.com/kb/291988

http://msdn2.microsoft.com/en-us/library/ms791558.aspx

So it is true, and important to know how to use it.
In some form or fashion 32bit vista can use 6gb in total. two gigs are relocated in some fashion to make this work.
The MS Article didn't provide enough info so I'm left with having to reply when they fix that. I filled the survey question
on that aricle so it would be corrected hopefully. Will post If I see a change.

Here's a link that states that the use of the PAE (Physical Address Extension) can enable 32 bit versions of Windows to use more than 4 gB of physical memory: http://msdn2.microsoft.com/en-us/library/aa366796.aspx



Following the BCD Edit link ( http://go.microsoft.com/fwlink/?LinkId=93291 ) leads you to a page that discusses the implementation of PAE when the system boots or with hot swappable memory (only available in Server OS's now). It sends us to this link that RandomUser already posted: http://support.microsoft.com/?kbid&ID=929605

That link discusses (in the More Information section) the problems with using PAE - and the consequent driver failures that may occur. And it states that DMA transfers and memory allocation are the most common problems caused by it.

Not a nice picture - but it sure makes me want to try it out. FWIW - the article also states that the maximum memory available will be 3.12 gB - on my system I've got 3.26 gB, so I'm not sure of the validity of it all.

I am reminded of the LIM memory tricks of the 80's. They allowed more memory in DOS, but never as a native actuality. The result was better performance for big spreadsheets and such, but not much else was helped. If you need more ram, then use 64bit hardware with an OS that will support it natively. Seriously consider something NOT produced by Microsoft.