I have recently upgraded my PSU. I went for the Corsair 750W TX for my rig. Its a great PSU, plenty of power for upgrades and is from a good well known supplier, plus the price is not bad at mo. I run my rig (i5 750, GTX275, x3 HDD's, x6 fans, x3 USB 3.0 PCI cards, 4GB RAM). I will be looking to SLi my GTX275 soon and the 750W TX can easily handle another GTX275.
BUT it does really depend on your upgrade plans for the future.