Vista - Vista shows 2.5GB when I have 3GB installed, but only 2GB when I have 4GB installed?

I have an asrock 939 dual sata2 motherboard and an AMD 64 3800 cpu, I have run xp fine for years but thought I would upgrade to Vista,
I got Vista Ultimate 32bit and then found about the ram limit on 32 bit OS, but for some reason I get more ram showing up in vista when there is less in the motherboard?
When i have 3 x 1gb stick in it shows up as 2.5GB ram, but when I put 4 x 1gb sticks in it takes it back down to 2gb (which is what it showed when I only had 2 x 1gb sticks in)
I then installed vista ultimate 64 bit and all 4GB showed up fine ( I did this to check it wasnt the ram that was faulty) I would have kept 64 bit but I couldnt find any wifi drivers to work so I have gone back to 32bit ultimate.
Is there anything I can do to get the right amount it should be when i have 4gb in?
Do you think the system will run quicker with 4gb in even though it only shows 2gb or should i take a stick out so it shows 2.5gb?
Ive looked on a lot of forums and it seems most people can get 3gb showing when 4gb is in so any ideas why its not working for me?

PAE
(physical address extention) and also the pagefile increases as the ram increases, solution : vista 64x

I had real problems getting everything i use to work on x64,so if I do stick with 32 I will be better off with 3GB rather than 4GB?
Or is there a way around driver signing on x64?