Advice on Small Upgrade

Hi just needing a few pointers on a possible upgrade I have planned but I've been out the loop somewhat since I bought my PC 3 years ago or so.

Existing PC

Core 2 Duo E6600 2.4 GHZ
4 Gig Corsair TWIN2X 6400 DDR2,
Asus P5B DLX
XFX GeForce 8800GTS 640MB
Resolution - 1680x1050
Cooler Master Wave Master Silver Case
Corsair Powersupply HX620W

Primarily I want to upgrade the graphics card as my 8800 gts is getting a bit long in the tooth now and struggles with games like Empire Total War and Napoleonic Total War, COD MW2 was ok but was still a tad choppy. I'm thinking of upgrading to an ATi 4870 or 4890 or a 5850. I'm thinking the 5850 is better in the long run (DX11, less power consuming than the 4 series?) along with a future price drop.

So If I upgrade my gpu to one of these will the rest of my rig limit the card (bottleneck? is the powersupply powerful enough?). Is it worth upgrading any other components to complement the graphics card. I plan to to get a new rig (i7/i9,DDR3 SLI/Crossfire) sometime in a couple of years but I need something that will tide me over until that time so I don't want to throw a load of £ on my existing set-up.

So my plan is

Ati 5850 ( I can crossfire this with another 5850 in the future)
Buy a CPU cooler and overlock the E6600 to 3GHZ.
Maybe add another 4 gig of Ram.

Does this sound like a good plan?

Any tips and suggestions are most welcome.

Hi I would say that getting the 5850 is probably the way to go, as you said it will run DX11 and is more power efficient than a 4xxx. It certainly wont struggle at your resolution.

Heatsink and a bit of an OC is always a good idea. Something like an arctic cooling freezer 7 is a good choice on a budget. Or an Akasa 965, both are much of a muchness and wont need you to rip your PC apart to install them, get whichever is cheaper.

I wouldnt bother with more than 4gb of RAM, esp if your primarily gaming. 4gb handles most anything you can throw at it.

Hope that helps.

Hawker