Need advice upgrading vid card, SLI

I built a comp a couple years ago with the intention of upgrading it later. With the advice of some friendly folks,
I opted out of two low end cards and got one higher end card, a geforce 470, with the intention of getting a second
eventually for an SLI setup.
They do not sell the 470 anymore, though, and I find myself wanting more power. A friend is willing to take my 470 off of
my hands, which should offset the cost of the new card(s). After looking a bit, I think the gtx 480 is a good fit for me, being
close to the 470, but still an upgrade, and reasonably priced.
I know I won't have heat problems with one card, I never did with my 470, but I am worried that I might encounter issues if
I try to SLI two cards known to run warm.
I have a Coolermaster HAF 932 case, and a ASUS M4N98TD EVO AM3 motherboard. I got this one especially so I could have an
AM3 processor, but still SLI (I've never used radeon, I go with what I know).

I would also like to know if you can think of any other problems I might encounter with this upgrade. This is the first computer I have built, and the first time I have upgraded it.

My other specs:
8gb ddr3 ram
amd phenom II X4 3.4ghz
coolermaster silent pro 1000w modular power supply