Here's what I would go with, as I figure you already have a computer you have the basics, a monitor, keyboard, mouse, and speakers. Either way if you didn't you could tack on the cheapo $6 logitech keyboard, $6 logitech mouse, monitor will cost you as will speakers. But if your fine with those for now, this comes out to $536 + shipping, to men the ship would be about $30 and you end up getting a free game, also comes with $30 of mail in rebates so you can get some cash back on this later.
I think this would be the best route, I feel the 710 is a better proc for the money than the 550 as they are both $99. Just when you OC this you will have to use the FSB instead of the multiplier. Also the 4850 is about the best videocard you are going to get for that price, $79.99 is a steal for a brand new onem yes it's only 512mb, but I don't think your a huge gamer so you most likely aren't going to be playing at crazy high resolutions. And this nets you a copy of Win Vista 64bit with a Win 7 upgrade for free, so your set there, if you already got a copy of an OS you could get this build for under $500.
http://img.techpowerup.org/090724/Capture001.jpg
Those actually aren't budget cards anymore, you can get a 4870 for the same price ($119.99) Theres a 4850 on the Egg for $79.99 after MIR ($99.99 before) and it comes with a free game.
The 810 is also $30 more than the 720, and the 720 is easier to clock and clocks better netting closer results when OC'd. A $150 proc on a budget of $500 is just too much, granted I think the 720 is a touch too much also at $119, which is why I tossed the budget screamer 710 in my suggested build.