Well, I know I'd be happy with standard magazine print resolution.
A DVD coverslick is 27.2cm x 18.2 cm
at 300dpi that reduces down to
3212 x 2149 pixels.
Your Zak McKraken front cover (very nice, by the way) at 100% JPG quality goes from 6312x8118 pixels to 1687x2149 pixels, the file size reduces to 2.4MB, and at 300dpi, the quality is absolutely fine. Saved at 95% JPG quality (indescernable from 100% quality when printed) it reduces in filesize by half again to 1.2MB.
While it's good to work in ridiculously high resolution, there is no need for the final uploaded version to be any greater resolution than magazine print quality - 300dpi.
Also, are you making full DVD slicks with spines and the back coming from the back of the original box, or just converting the front pictures of the boxes?
Also note that the original author of this thread was thinking 300dpi resolution when he said "ridiculously high"
(by ridiculously large i mean at least 2170 pixels high. No, really.)
21 pixels difference to my proposed final size - negligible. Just due to the DVD slick I measured being a mm or so different to the one he measured.