Basically, my PC has 4GB installed plus 512 MB of VRAM but Windows XP only recognizes 2.5 GB. I know the whole 32 bit RAM limit thing but there's no way my PC has another 1 GB of physical memory. Anybody know why?

its _impossible_ to use 4GB of ram on windows 32bit.. even if it reports all 4GB of it.

with PAE (physical address extension) enabled you can have windows report the full 4GB of memory but it won't be able to use it still (since drivers need to be PAE aware else they crash due to pointer sizes being 36bits large vs 32bits large.. so PAE is purely there to enable DEP in windows XP.. Pre SP1 it did work properly but yeah drivers sometimes crapped out). PAE needs to be enabled in the bios.

All devices share the same 32bit memory addressing space

so at best you have
4GB - Windows 300mb odd reserve - Video ram, that should leave you with about 3.2GB of ram, meaning you must have some other device eating up the rest of the ram or addressing space reserved (what other devices do you have installed?).

Considering regardless of what you do you'll lose close to 1GB of ram, if I was you I'd seriously look into upgrading to 64bit XP or Vista (depending on which has better driver support.

In other words, we don't know why it's only showing 2.5 instead of 3.2.

Yeah, I figure XP would recognize somewhere like 3GB of RAM after that's all said and done but I don't.

My PC is pretty basic: A motherboard, PSU, C2D E7200 CPU, 8800GT, 4GB DDR2 RAM, Hard Drive and Lightscribe DVDRW both using SATA, stereo, keyboard, optical mouse, and USB wireless adapter. I can't figure why those things combine would restrict my RAM to just 2.5GB.

Edit-yeah I know about PAE but I don't care about it and changing an OS is not so great. I just can't figure out why the OS won't recognized at least 3GB.

bingo hence I asked what other devices he has installed.. Most devices use an addressing range.. if he has some funky sound card with its memory mapped it cound explain why.. it could also be a bios problem.

Do you have 4*1GB or is it some other combination?

totalwar23, whats your Motherboard details, chipset and bios version?, also does POST show 4096MB of ram during memory testing?

how much ram does your video card have?

and yes, it does recognize the 4GB. even CPU-Z recognizes 4GB.

ssj12, he has 512mb, hence it isn't adding up

let me check hunt for a manual for the motherboard.. you might not have the correct memory configuration (ie ram in of different sizes sharing the same channel).