Please giv details of the Direct-x version that ur using.Also try updating ur Video Card drivers from the manufacturer's site along with the drivers of ur motherboard.
Please note that since u do not hv a powerful AGP card,so playing DOOM-3 at a resolution of 1024x768 will be really a foolish idea(please dont tak it as a negative remark,bro!) ,so please try playing at screen resolutions of 800x600 or lower with the visual settings set to low or medium like the shadows and other graphic rich lightening effects as it takes up a hellofa SYSTEM RESOURCE.So distribute ur graphics properties wisely.....

as the amount of RAM is almost sifficient for ur game play..(but as ppl say that no matter as much of RAM u feed to ur PC,its hungry for more.....

)....
cheers n e-peace....