Asus A8N-SLI, 4G Mem, XP Pro SP2

Hey everyone. Did some searching on the net about this but found all kinds of different responses. Not sure what to do. I just upgraded my video card, PSU, and Ram. My current specs are:

Asus A8N-SLI
AMD X2 4400
EVGA Geforce 9800 GX2
4GB OCZ PC3200
Sound Blaster X-Fi
Dual WD Raptor 150s in Raid 0

Now in system properties I'm only showing 2.5 GB instead of 4. I know with XP you can't see all 4 but others have seen as much as 3.5GB. My bios is enabled for remapping with 4GB and when I'm in the bios it shows all 4gigs. How do I get it to show more in XP? I also have most settings, including the ram speeds, set to AUTO in bios but the default max is 333mhz when I belive the max is actually 400mhz...but doing it manually calls for a bunch of other settings to be configured. Does anybody have a good bios setup for this motherboard with similar specs? Thanks in advance for your help.

Nope.....still showing 2.5GB. With Vista 64 I showed 3. My friend has vista 64 with 8gigs and he shows all 8 so i don't know. I show 4GB in the bios.