go to the manufacturers web site and see if there are any drivers available which might need to be installed (though windowsXP should be ok with the default drivers) it might not be the ammount of memory that is the problem as much as directX, a few games demand directX 9 and some cards do not support that, check your card is directx9 compatible and see if the game spec mentions directx 9.
i had this problem with Broken Sword 3 and had to buy a new graphic card to play the demo (strangely though, once i bought the game it only needed directx 8).
i have had a quick look and if this is your card "Radeon Atlantis 9800 Pro 128 Mb AGP" then it reprts as supporting directx 9.0
the following is from a review site about the game F.E.A.R. supports the DirectX 8 rendering path, but we highly recommend you purchase a DirectX 9.0 video card to really take advantage of the game. If you play F.E.A.R. with a DirectX 8 video card, you’re really missing out on many of the features that the game has to offer. It’s almost like calling wiffle ball baseball. The game is the same, but you know it’s just not right. data from click here
hope this might be of some help to you