Perfect, you got your overclock to exactly what I had in mind
Well done sir. And yea that's probably about as far as you're going to get with that chip, a very respectable overclock.
As for the PSU, I see no need to upgrade. Even a GTS 250 only consumes about as much power and an 8800 GTS 512MB which I used in combination with a Corsair 450VX on a buddy's rig with no problems at all. As long as you don't get a version that requires two 6-pin PCIe connectors, that Corsair 450VX should be able to handle any single GPU card. Heck, I'm running dual HD 4830's on a 550VX with a heavily overclocked 65nm quad core too and I have zero problems. These Corsairs are beasts
The 450VX is equivalent to what most manufacturers would brand a 500W PSU. It's rare to see a single GPU system break out of the high 300's in power consumption. Most hang out around 350W-400W. Even this dual 6-pin version of the GTS 250
had a FULL SYSTEM load of 362W, so a single 6-pin version should consume even less.
And remember, as long as the card has only a single 6-pin PCIe connector, it is impossible for the card itself to consume more than 150W, 75W from the 6-pin connector, 75W from the PCIe slot. 8-pin cards and cards with dual 6-pin connectors can consume a max of 225W. So when people upgrade from an older 6-pin card to a newer 6-pin card, they tend to think their power needs are going to skyrocket when in fact they're usually staying about the same, maybe an extra 10-30W.