The D200 and the D300 have built in AF bodies. There are others, but I'm having a brain cramp at the moment. An AF body gives you more lens options than a non AF body.....that's really the only difference.
Yeah Jack is right the D200 is a great body, I have one and it is a ready great camera. You should be able to find one fairly cheap as well. You may like it better then your D90...lol.
Don't forget about battery compatibility too. I know that the D7000 battery is different from the D90 and the D700 (700 and 90 use the same battery). 2 different batteries = 2 different chargers… Not so nice.
That is very true, I have a D200 & a D80 which take the same battery/charger.
My D7000 uses another type of battery, I might add the D7000 holds a charge way longer (so the new battery is a improvement) Its just too bad its a different type and does not work with my other bodies. But no big deal...lol.
I know Curt that it's not a big deal, but I find it slightly annoying to have about 2 or 4 different battery chargers around the battery charger dedicated drawer they could standardize these things as far as I'm concerned.
Lol... you think that is bad, I have a Canon G10 & a SD900 both take different batteries as well, plus a JVC camcorder.
So I have a half dozen chargers around my place .
In addition to the battery issue, there is also the memory card issue. IIRC, the D90 uses SD & the D200/D300 are CF; multiple batteries/chargers, different cards, definitely something to consider.
D1, D2, D3 all have AF motors in body, I am probably wrong (I usually am) but isn't it the D40, D60 and the thousands, apart from the D7000 that don't have inbuilt AF motors?
I can tell you that the D2x is a brilliant bit of kit, uses CF cards and fits the elen4 battery if that helps in any way. Good luck with your second body