a 460gtx 1gb sits in a hole AMD don't really have anything, only the 5830 which is a heavily cut down 5870 so its basically a very expensive to make chip for AMD, it just doesn't work at a low cost.
it would go, in terms of speed top to bottom 450gts>5770>460gtx 1gb >460gtx 1gb
with prices around £100>£115>130>170 at the moment.
Now when the 6series launches things will change, the 5770 will likely end up in sales, probably below £100, AMD will likely have a new 5770 with +25-35% performance at the same price, which will likely put it pretty close to/above a 460gtx 768gb.
AMD are set for a pretty large shakeup it would seem between around £80-180 so its well worth waiting till end of October, unless you have a game absolutely crying out for something in the next month its crazy to not wait.
The best case scenario is theres a new card thats insanely fast, stupidly cheap and blows all of that lot away, worst case scenario is theres a new card thats a little faster, which pushes the prices of ALL those cards down.
Right now a 460gtx 768mb if you can find one at £115 is easily the best value under £150, they were on sale recently with lots below £120 like the one I listed, its a VERY decent card for the price, even after the new cards its unlikely to drop below £100 so theres not a whole lot more to save if you can't wait.
Power wise on a decent quality 520W, just about the only card I wouldn't go for would be a 480gtx even though it should be fine, if you overclocked/overvolted it(easy free performance) the power usage goes up quickly and with 280-300W at stock, thats going to be pushing it. Basically any card you buy sub £300 your PSU will be WAY more than you need.