Been gone a few days, but since nobody else jumped in to answer your last question ...
Yes, removing the single 512mb module should cause the system to switch to dual channel mode (I would definitely do that).
Everest is indeed a very nice utility :-)
It's certainly perplexing why your system runs SLOWER with the increased memory ... do you get the same result if you remove that 512mb module??
... and Yes, I'd think it would run quite nicely with XP.