Through trial and error (and a few hints from the forum) I have learned a few tips for those who wish to download a fairly large area.:
1) Set map detail to 'lowest'
2) Do NOT use 3D mode. Use 2D when dealing with areas that have birdseye imagery.
3) Do not try to zoom in and out of imagery areas with the mouse wheel or by clicking the +/- on the map controls... this tends to cause Basecamp to crash. Rather, use the slider to set the zoom to where you want to be. Basecamp may still crash but your odds will be improved.
4) Plan ahead... it will probably take you DAYS or WEEKS of screwing around and restarting BC to get everything loaded. ... It does work eventually and who would think... the tiny little processor in a handheld GPS works great with the Birdseye images that nearly cripples Basecamp...
These tips have allowed me to get the non-responsiveness of BC down to 10-20 seconds between screen refreshes in ares containing multiple hi-res Birdseye sections (with out these it can take minutes between screen updates). It also drastically reduced crashes while downloading birdseye images. It still crashes every other map upload to the GPS though.
(I am running BC on a Dell M1330 with 4gb of ram on 32bit Win 7 with a Garmin Oregon 450).
P.S. Birdseye maps work fine when loaded to the SD card in the 450