Oblviion is coded for dual core ...so dual-core obviously
Also in anands tests dual-core scored on par with an FX 200mhz faster. Now that's with an absolutely minimal OS and no sound (I think) which would offloaded to the other core.
Add in a full loaded OS with bloat and many background processes and than dual-core will be doing even better.
Also, nvidia and others are looking to offload CPU work onto the second core making it faster even for single-core games.
Dual-core is the future so is worth more in the long-term where it has increasing value while the FXs value only decreases comparatively.
In short X2 = the winner.
There's no reason to chose a FX over an X2 - X2 is just plain the better chip regardless and you should want to get it.