I believe the GF9800 is just a 9800GX2 card. These cards perform excellent, but are extremely hot, consume a massive amount of power, have driver issues and cost quite a bit. The ATI 4870 is a great card on the other hand, with none of the above issues. While the performance is not quite as good, it's still a worthy card, even today.
Google searches show that GF9800 is an incomplete product name. There are GF9800 GT single cores with 1gb of memory, as well there is GF9800 GX2 dual core cards with 1gb of memory.
If it is a single core Gf9800 (GT), it would be safe to say that the 4870 will best it in most ways, but require more power.
If it is a dual core (GX2), it would provide more raw power than a 4870, but that 1gb is split into 2 parts (since multi GPUs load the exact same data into memory for each core, the nature of multi-card setups) So you may actually have more resolution limitations using a GX2 over a 4870. In lower resolutions (1680x1050 or lower) the GX2 would probably outperform the 4870, in theory. But any higher (1920x1080+) there may be better performance to be had from the 4870.
As mentioned, The GX2 will use more power than a 4870, though it may not be much. It will probably be hotter than a 4870, though the 4870 is a warm card on its own.
It's too bad the 9800GX2 had those flaws, it would have been a great card otherwise. So in theory, go for the 4870, if those are your choices. You may want to explain more to us if you are thinking of buying something.