2 GPU Q's

soem amd boards do support sli but most of the newer ones do not. so honestly depends on the board. the 450 is superior to the 9800 gt

http://www.tomshardware.com/reviews/gaming-graphics-rad...

2 tiers higher a good gain, the 450 is a good card, its what i use. it doesn't have a much muscle as the rest of its older fermi brethren but depending on your resolution and games you play (and often budget) a 450 can be a great choice

But what im asking is does it support just one Nvidia card? (im not going to get another for SLI)

yes, as long as it has a pci-e slot it shoudl support one nvidia card provided it is also pci-e

In other words you could use any nvidia card on your mobo as logn as it has a pci-e slot