If you can do both, of course do both. However if your funds is severely limited, I would recommend upgrading the graphics card first. You will see substantial improvement in gaming.
as for the GPU, the 4870 will beet the GF9800GTX+ hands down, so this would be the most benificial upgrade out of the 2 stated, although the GTX260 maxcore, is coming down in price quite nicely, making it a good choice against the 4870.
if u can upgrade both then even better, but if can only do one at a time then i would recommend the GPU upgrade first.
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