I was looking at the side of the box my comp came in and it it is meant to have 512MB DDR-SDRAM. But on looking at my system info it says, 984 MHz, 488MB of RAM physical address extension.

Can i ask why this might be?

Guess No one knows.

I was hoping you could help me try and unlock the remaning RAM

This article may give insight into the issue.

alright stupid SP2 :P

Is /pae in your boot.ini?

This could also be where your BIOS needs to be updated.

not sure bought the comp from Pc World in the uk?

it came with a google toolbar installed lol

Well, being that you don't have 4GB of RAM, that article is irrelevant (I guess you didn't even read it, Tarun! It repeatedly talks about 3 & 4 GB of RAM ).

Anyway, unless you have a separate video card, then your motherboard is using system RAM as video RAM for its onboard video chipset. It's using 24 MB right now. If you have an intel motherboard using one of Intel's latest graphics chipsets, this number may fluctuate up and down as your system demands more video RAM. You also may be able to adjust the amount of RAM it uses in system setup/BIOS (don't reduce the amount though).

the processor is AMD ATHLON 64 3400+

Graphic card is an ATI RADEON XPRESS 200 GRAPHICS

Graphic card is an ATI RADEON XPRESS 200 GRAPHICS

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This article talks about your "video card" (which isn't a card at all):

http://www.ati.com/products/radeonxpress200/index.html

The last sentence of the last paragraph states:

ATI’s high performance integrated graphics processor eliminates the need for a discrete graphics card for a cost-effective solution.

Like DjLizard said, your drivers are simply telling your computer's RAM to act like a video card. However, you can potentially get increased video performance by raising the amount of shared memory. I have never used a computer that wouldn't let you adjust the shared (video) memory with the BIOS. This is a nice feature to have, because if you ever buy more RAM, you can raise up how much memory is used for graphics.

If you do decide to increase the amount of shared memory, I personally wouldn't suggest using more than 64 MBs for video, since you only have 512 MBs of main memoy right now. If you use too much, your PC may actually lose performance. But as Dj also stated, you also don't want to have too little dedicated to video. Between 32 and 64 MBs is a good amount for most users, but it totally depends on what you use your computer for.

So Basically i should not modify anythink.

Right.

You will want to upgrade your RAM to 1 GB in the future though.

Will try and do... :P