Your board has a PCI-E x16 slot. The size will depend on your case; top-end cards are quite long. They also have as many as two auxilliary power connectors on them. What brand and model is your PSU? Does it have a 6-pin PCI-E power connector? If your power supply is small (e.g. 250W-350W), you will be limited on what card you can install, unless you replace that too.
Do you play games? Which ones? If you don't play games, you really have no reason to get anything more powerful than a 4350, which is $35-$40 now. If you play games, the strongest you can get with a weak PSU would be a 4670, which needs no auxilliary power. Those are about $80 now.
Oh, I didn't mean to sound like an ATI fanboy; that's just what I've been using lately. nVidia has cards that would be suitable as well. HD4350 ~ 8500GT or 9400GT; the 9500GT or 8600GT are about the strongest nVidia has that don't need additional power, but the ATI HD4670 blows them away.