i think gt240 is the better choice. this guy is clearly on a budget and the gt240 is less power-hungry than the 9800 gt but performs about the same.( check out the Tom's Hardware review) getting a cheap 400-w psu will probably be enough for the gt240. plus, the version you're looking at has 1gb gddr5 memory, which is much higher than the 9800 gt's 512 mb of ddr3 memory.
The GT 240 is slightly weaker than a 9600GT and not "about the same" as a 9800GT. A cheap 400w can handle the low 9800GTs as well. The difference in memory speed is almost entirely erased by the different size memory buses and the amount of memory is pretty much entirely irrelevant for cards of this caliber.