To get into download mode when all else seems to have failed:
Step 0: Unplug USB cables, start Odin [so you are ready to fix your problem, if things work]
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1) Pull out the battery (to ensure that the really phone is turned off)
2) Reinsert the battery.
3) Without pressing the power button, simultaneously hold down both sides of the volume rocker (volume + and -)
4) Plug the USB cable into the computer USB port
5) Plug the USB cable into the phone's USB port
If you see the familiar yellow triangle, or the weird Gingerbread "firmware upgrade fail try using recovery mode", you're now in download mode.
6) Reflash
Hope this helps, if it did, consider pressing thanks - it makes me smile.
AoN