It looks like your motherboard would support PCI-e X16 but you are very limited as to what you can get with only a 305 Watt power supply. You might get lucky with something like the Nvidia 9600GT but there is a chance that 305 watts might not be enough because the recommended watts is 400. Just search around on google and newegg (or maybe someone else on Tom's can help you out ) to see if it might be possible, and if not go for something a little less power hungry.
dont pay attentino to the recommended watts... Dell PSU's are generally pretty good. a 9600GT should run no probs on a dell 300w. i've seen cases like that all over the web. i think someone even ran an 8800GT on a dell 300w.. meh.
4670 could be good though, since it doesnt need any pci e plug from the PSU