^ rule of thumb... old nikons (before D3/D300 release) are pretty bad & forgettable dSLRs you want to avoid. Old canon dSLRs (any) are still very, very good
the 1DmkII advantages over 50D are significant. (i sold my 5D soon after picking up a great priced 1DmkII almost 2 years ago now).
* much larger sensor with less crop 1.25:1 vs 1.6:1 crop vs 35mm sensor
* much better (still unrivaled) auto-focus (AI Servo or one-shot)
* more AF points and better placement of them
* much better metering & thus exposure
* ability to lock spot-metering to a non-centered AF point (spot meter any selected AF point).
* ability to lock metering on different point than AF point
* more custom controls
* built in grip
* battery that lasts 4x longer
* 100% viewfinder
* more info in viewfinder including flash compensation & exposure meters
* Raw files hold up better when pushed in photoshop/lightroom
* built like a tank
* you don't look like a tourist in a photoshoot

* high ISO of all these cameras is overrated.
* can use 2x extenders and still AF on slower lenses
The only advantage of the 50D would be when you are zoom challenged in GOOD LIGHT. For birding or safaris where you need long zooms and the 15mp 1.6x crop body would provide a longer zoom and also more crop-ability.
Sure you can buy 1DsmkII (full frame 16.7mp) for under $2k but the 50D Steve won't use e-bay to sell would be lucky to t fetch $500 on craigslist and the grip is equally as worthless to a "craigslist" bargain basement buyers... glen