Unfortunatley, the P5K isn't an SLI motherboard, as it would need an nforce chip. It can only do Crossfire setups i.e. multi ATI GPUs, so unless you upgrade your motherboard, then SLI isn't an option, sorry
A GTX series will still be a great upgrade, though. If the 275's price is your price range, then a GTX260 or 275 would both be a good choice, though I've read and heard the 275 gets a bit too hot when under load.
My two 8800GTS 512s in SLi was recommended a 700w PSU, so you might have been pushing it with a 550w (I rememebr trying a 500w on my 2 and it shut down when under too much load). It *might* be okay for a GTX series card, but don't hold me on that one