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At the risk of sounding "old-school", IMHO your best option is to lose the flash header altogether and use a different theme. In the first instance, flash can cause accessibility options, especially for users viewing your site from a webcafe or library for example, where a flash player isn't installed. Secondly, a flash header, especially of that size, is going to use a lot more bandwidth than you might want. Thirdly, a site optimised for 1024x768 with no resize option is also going to cause problems with certain browsers, each of which have different display sizes.
In my opinion, it's always best to ensure a site looks good on an 800 width screen first, then think about the higher resolutions.
I know this doesn't answer your question, but I hope it has at least made you think about how important that flash header REALLY is.
Regards,
Gareth