Another vote for splitting, I've shifted on a couple of old comps that way in the past and have always made considerbly more than what the whole system was worth as a unit. Also a dansite easier to shift stuff that way.
Bundle the Mobo, CPU and Ram together. The "drop it right in", "all in one upgrade" bundles always used to do well on the bay and probs do ok on hexus too aslong as your sensible on price.
Case and monitor should do well wherever you sell them as they're quality items. Rest of the bits and bobs, might not be cutting edge, but you'll be surprised. I've gotten £8 for old DVD drives at carboots!!!
Only issue is your XP licenses, dont think they'd add any value to any component as no one looking at this kinda stuff is likely to be XP'less, may have to write it off as the joys of OEM software.
Good luck with whatever you do anyway.

Lemme know if you do sell seperate as few bits there I might be tempted by, but will pass on the cookie as I'm a tree hugging veggie
