First, MikeFellh is right about "Full Frame TRANSFER...it does not mean full frame sensor (that would be a sensor size of 24mm x 36mm as with the Canon EOS 5D). The E-1 has a 5MP sensor and the E-500 an 8MP sensor (same size @ 17mm x 13mm).If you're only into the numbers game it's a no-brainer. I own two E-1's, buying both after the release of the E-500. Pro body, weather-sealed, twin mode dials (I can't deal with just one, personal preference) and a much bigger & brighter viewfinder that the E-500. 5MP does the job for me quite well.
So the Pentax won't focus on the center ofa well-litplain blue card but will at the edge? You've just described a number of DSLR's (including my E-1's). Contrast-detection focus system. No contrast, no focus (there's contrast between the card and what lies beyond the edge, though...see the logic?) In low light when the subject is close enough the E-1 will emit a focus-assist light, basically a series of red cross-hairs projected onto the subject providing the needed contrast to focus on. Not sure if the E-500 offers it; check the specs.
Finally, the E-1, if you can still find one, is now no longer more expensive than the E-500. Goes for @ $465 US$ body-only on ebay from Cameta or Olympus auctions. Fire-sale pricing is whyI bought a second body for backup.