Crossfire x16+x16 vs x16+x8 vs x8+x8 benchmarks?

I am having to maximize costs of a gaming rig I am building for a friend and was wondering how much FPS difference there is between x16+x16 and any lesser config of crossfire. One stands to save 100 plus $ if there is only a minimal difference, seeing as dual x16 boards are signif. more expensive than dual 8x or 16x+8x.

I'll add, since it took me awhile to understand the results of that test, that there appears to be minimal difference between dual x16 and dual x8. The relevant chipsets are the X48 and P45, respectively.

For that extra $100, you gain barely 5% greater performance from the dual x16, and that is only on certain games at certain settings. On some of the benchmarks they tested, there was no difference between dual x16 and dual x8 at ultra settings and 2560x1600 resolution.

In other words, I will be saving a lot of money in a couple days and going with a 790GX rather than 790FX chipset. Instead, I'll put those bucks towards a HD4890 instead of a HD4870.