I think you mean Airport Extreme, not Express. Airport Extreme is Apple's term for 802.11g, Airport Express is the tiny Airport base station that you can use to stream music from your computer to your stereo.
All computers Apple is currently shipping include Airport Extreme (it's optional on the Mac Pro, standard on every other model).
The current iMac and MacBook Pro appear to support the DRAFT 802.11n spec - however, because it's not been finalized, this could change. Also, several vendors seem to have different, incompatible 802.11n spec hardware. It is likely this will all be worked out, and current hardware will be updated to the final spec with a firmware upgrade, but there's no guarantee.
Finally, Mac OS X does not yet support 802.11n, so you won't really see any speed gains, unless you install Windows and find a working driver.
If you're in the market for a router, I'd probably buy an 802.11g router, and wait until the 802.11n spec is ratified, and hardware becomes more mature.