I have 8 gig but only 2.5gig showing, help plz

You did not tell us what operating system you were using. I'm using you use a 64-bit OS? If so, you may need to enable memory remapping feature in BIOS.

That's why, its 32bit. In order to make use of more than 4gb you need a 64bit OS.

2GB is the usual limit for 32-bit, although with PAE you can extend the memory; but still with limitations. In no instance can a 32-bit application address more than 2GB (unless its compiled in a special way allowing up to 3GB addressing).

Consider a 64-bit OS if you run more than 4GB RAM.

XP pro 64 has driver issues.

You need vista 64 or windows 7 64

ok, that answers my question

Can you tell me what games use 8GB? Especially since like all games are still 32-bit so can only use 2GB maximum....

You might be best off installing a version of Windows 7 64-bit.

More RAM would allow the game to store more (like bigger levels/environments). But that wont happen right now since games are only 32-bit and 64-bit games will not become common until amd64 is common, which may even happen after Windows 7. IMO it was a mistake to ship Windows 7 32-bit; it could have been a great moment to go 64-bit only.

But as people at home are still using 32-bit operating systems, there sure aren't going to be many 64-bit capable games out there that can use more than 2GB memory. 1) not many games need so much RAM and 2) too few customers have 64-bit capable OS so it will not be commercially attractive right now.

They should have never made a 32-bit version after XP, that would have helped.

Because your video card and other hardware are mapped onto the 4GB.

You cannot.

I was thinking that no matter OS you use, if it's 32 bit it will only adress max 4gb, because of "the 2^32 thing" ???

The best thing the OP can do at this point is simply return 4 gig of the memory, or sell it to someone else. Right now, there is no reason to run out and buy Vista.
It is going to be obsolete in a few months with the official release of 7.
Don't waste money on buying Vista. It is simply not worth it. Just keep your XP pro for now.
XP will not use all the memory if you have 4 gig installed, but that is okay. It will still make use of all it can. The extra doesn't hurt anything. And later, when you do decide to upgrade the OS to 7, you are set.
But to that end, you still don't need 8 gig. That is simply overkill, and you'll never need that much.

[quotemsg=1817233,23,95014]sub mesa your data is a bit off, as subAnimal pointed out. 32-bit Windows can address exactly 4GB of memory space.[/quote]
But applications cannot. They are limited to 2GiB. They can address up to 3GiB if they are specially compiled with /LARGEADDRESSAWARE and /3GB flags, but very few applications do this. Games like World of Warcraft don't, in any case. So their actual memory space is limited to 2GiB, not 4GiB.

The OP was not asking about applications.

Right, even though with more memory(not saying 8gb) a game may only use so much, but what about the other programs running in the background and the OS? they need RAM too.

My experience has been, with 2gb, Alt-Tabbing out of a high memory usage game is a pain but still works albeit slowly. With 4gb its much much quicker.