A used 5D mk1 sells for about half the price of a used mk2. Rather ironically the biggest difference between them isn't resolution though you would think that to the case from the specs. On screen the files from the Mk2 are much better but this all but disappears in print. The mk2 also has much better performance in the ISO 400 to 1600 range, but below 400 the mk1 is pretty much the same.
As has been said, the digi-2 is pretty slow and there is no video or live view. However having said all that the mki is probably the bargain out there now in Dslrs. IQ wise it is certainly current up to 400 ISO. At Building Panoranics we thought it was better than a recently aquired (and now sold) 650D.
If you get one go for one thats serial number begins with 2 or 3. They have the improved screen and modified mirror. Avoid ones that begin with 0 or 1.
Good to know about the serial numbers. Thanks!