Is there any reason to spend more for a MB with more than one PCI Express 2.0 x16 if I plan on using only one 8800GTS-512 GPU?
 
Are there any other cards that need this type of slot?

NO. Simple as that, even if you want to SLi. I'd still discourage it due to stability issues. Go Intel chipsets or AMD chipsets. You don't need PCI-E 2.0 yet. 1.0 or 1.1 is fine. I'd worry about putting a GTX 280 into a 1.(1) slot though.

Most relatively newer boards come with PCI-Express x16 2.0 slots anyway don't they?
 
You could keep in mind that it all depends on how many PCI-Express cards you plan to use, whether they are video cards or something else.  Most boards that I've looked at that only have one PCI-Express x16 slot usually have a couple of more PCI-E slots (x1 or x4) and then 2 or 3 regular PCI (non express) slots.  You can use lower PCI-Express cards in higher PCI-Express slots.  But you can't use a PCI-Express card in a regular PCI slot.  But generally, most things come onboard now and there's no immediate use for PCI-Express cards except for video cards.
 
So as long as you don't have too many PCI-Express expansion cards to utilize, you won't need more PCI-Express x16 slots than you do video cards.

Yes there is
I wanted an X48 mobo with 1600 FSB support and DDR2 and OC easily
I found Asus Rampage Formula but it has 2 PCI Express 2.0X16
I can Cross Fire but I do not intend to  
So I ended using one 8800GTS(512)
and E8500 OC'd to 3.8 ( 9.5X400) i.e working at 1600FSB easily
And I'm so satisfied

you can still buy single pcie boards the P45 chipset is "reported" to have 1 pcie 2.0 slots.
 
these boards are usually cheaper to purchase. but dont be put off a board just because it has an extra pcie slot
 
the 3 way sli boards are quite pricy and not worth it unless you are buying 3 8800gtx / 280gtx