The OS X 86 Project is reporting that Rosetta now supports Altivec in Mac OS X 10.4.3 for x86. Previously, Rosetta could only emulate the G3, and thus any PowerPC-only application that required the G4's Altivec instructions to run would not run at all through Rosetta on OS X for x86. If Rosetta has now gained the ability to dynamically translate Altivec, that would be a huge benefit during the transition period. While Altivec translation is bound to be slow, slow functionality is better than no functionality. This claim of Altivec support is unconfirmed however, and Apple's developer site still states that Altivec is not supported. (Note that such support is not required at all for the iApps. All the iApps, including iTunes, now apparently have universal binaries with native support for x86.)
The same article also states that ATI video cards for PCs are now also supported, in the latest seeds of OS X x86. If true, this would reduce the need for dedicated Mac video cards. People could simply go out and buy the latest and greatest PC video cards at the local video store, and plug them right into their Power Macs. Presumably any such support would be best on video cards utilizing reference designs, but the selection of PC video cards based off reference designs is still bigger than the selection of Mac video cards. However, this claim of PC video card support is also unsubstantiated.
We may find out the truth in a few short months though, as x86 Macs are (hopefully) coming relatively soon.