Agree with VincentP, the current processor is quite weak by current standards and only meets the minimum requirements for quite a few of the current games.
Obtaining anupgrade is possible, the motherboard supports a wide range of processors. Newegg, Frys and NCIX all show limited stock of suitable upgrades, but be aware, you'll probably need to flash the motherboard BIOS beforehand.
This list is from the Asus site and shows which processors the motherboard supports:
http://www.asus.com/Motherboards/M2A74AM/#support
The current motherboard will support all available PCI-E cards, assuming they'll fit inside the case that is.
High end cards need plenty of power and will need at least dual 6 pin PCI-E leads to function, it is unwise to run a powerful card by using molex/PCI-E adaptors to power it, so please provide the full make and model of the installed power supply.
Unless the 9800GT is one of the 'green' versions it'll need at least one 6 pin PCI-E power lead and, given the 'weak' CPU I'd suggest a GTX650Ti or HD7770 GHz Edition as upgrades if you can't afford a CPU upgrade just yet.