Your computer has one available PCI-e 16x slot which is not occupied as the motherboard is currently using an onboard graphics solution. While you should just be able to stick in any PCI-e compatible card, your power supply is relatively weak being rated at only 250W. At that wattage, I really cannot recommend adding a discrete graphics processor as most that are relatively modern will require around 350W minimum with 80 PLUS efficiency. Pre-built power supplies are usually very conservative and overrated with poor efficiency to save the manufacturer money, you'll need to replace it before adding anything more powerful than 4650 or 8600 GT.
There's special holiday savings on a 380W Antec EarthWatts power supply with an 80 PLUS BRONZE rating
for $60 with free 3-day shipping which means it is very efficient and will handle power output over its rating if necessary. Once you replace your power supply you'll be good to go, though I would recommend just getting a 512MB 9800GT since it is not powerful enough to use the 1GB of memory, nor would I guess your resolution be high enough.