SLi question

ive been thinking of going sli - and ive desided i will this year and ati crossfire cost to much.

i was thinking of this mother board
http://reviews.pimprig.com/motherboards/dfi_lanparty_ut_nf4_sli-dr.php?page=1

the questions is the cards now i know they have to be of the same type so for example 2 6800, you cant have 1 6800 and 1 6600 (any exceptions to these rules?)

what i am not sure on is could i have 6600gt and a 6600ultra? or would they have to both be gt or ultra? if you could mix like that would it give a decrease in performance?

now about sli motherboards (yes i know nothing about sli) can i still just use 1 card (say i dont buy the 2nd one at the same time) and it will run fine, i wont get a performance decrease will i? and then stick the 2nd one in at a later date

acording to the review of that mother board they use two 6600's and got 100+ fps on doom (along with the same ram and processor i have now) could i realy exspect that kind of performance of to 6600's? and a simular if not more in cod 2?

i currently have a x800 (agp) and as such think its time i move on

thanks in advance - lozz