You should notice a fairly big difference, it's a full x16 1.0 - which is the same as x8 2.0.
The CPU you're running is going to be what holds you back. You honestly don't even need to upgrade the rest of your computer, just get a decent cooler and OC the Q6600 to whatever you're comfortable with (it should reach 3.5GHz without much trouble)
Some rumours say nvidia cards will arrive late november....I think it will be longer though. And that will be the flagship card which will be overkill compared to the rest of your system. You will have to wait longer for mainstream versions.
I would say your only gain from waiting would be a potential price drop on the 5850 when the nvidia cards hit. Like I say, if that's the route you decide to go be prepared to go a few months with what you have.
@ kabobi: Your rig still has plenty of life in it, and will greatly benefit from an overclock, (extending its life and helping to fatten your wallet;) ).
If you plan on a major upgrade within a year I would suggest you hold off a little while: I am already seeing discounts on the 5xxx cards here in the UK and that will depress prices on the 'old' cards further. With a little patience you could grab a HD4890/GTX275 card as a 'cheap' upgrade and keep the system for another year that way when the big upgrade time arrives all the DX11 hardware will be widely available, cheaper and with bug reduced drivers.
If you plan on upgrading sooner then the HD5850 is the way to go, just carry it over onto the upgraded system.