how much do you wish to spend please?
just had a quick google for the rest of your specs are these correct?
Intel Celeron D 330 (2.66GHz) Processor 512MB PC3200 DDR Memory
80GB 7,200rpm Hard Drive 48x CD-RW Combo Optical Drive
Intel Extreme 2 Integrated Graphics with 64MB Shared Memory
any budget graphics card should play FS04 much better than your onboard graphics can but you may need still to keep the fancy effects and details set to low to get it to run smoothly in large airports. £30-£40 gets you an nvidia FX5200 or radeon 9250 for example. this is an old directX 8 card but still beats these budget cards if you dont have much to spend its worth the money click here
£70 or so should get you a radeon 9600 pro or similar card like nvidia FX5700 if you want to get something a bit better. this should allow you to play smoothly enough with the details turned up a bit.
£120+ gets a really decent card.