My opinion is to simply go with a Radeon 4870 it can be found for around the same price as the 9800gx2 and I believe offer comparable performance. the 9800x2 would be more powerful in some games but it relies on sli profiles to exist for that game. I know that my experience with the 7950gx2 was less that favorable because of this but sli has improved since then. I still recommend going with a radeon 4870. You could even crossfire them if you wanted more performance your psu should be powerful enough
If you can get a 9800GX2 cheap, then go for it. It is in the same class as a GTX260/280 and the 4870. Some games will run better on one card than another. Unless you have a favorite game you should be happy with a 9800GX2 in general.