ATI 7850 with last generation C2D or C2Q

Hi guys,

First of this is my current config -
CPU: intel c2d E8400 @ 3.00 (stock)
Mobo: XFX GeForce 9300
(http://reviews.cnet.com/motherboards/xfx-geforce-9300-motherboard/4507-3049_7-33428357.html)
GPU: XFX 4850 512mb @ 500MHz (under-clocked to get rid of BSOD)
RAM: 4GB DDR2 @ 800 BUS
PSU: TT 400w Lightpower
Display: ASUS MS 228 LED @ 1080p

I am going to upgrade my GPU. I want ATI 7850 (with TT smart 550w).
I know for a fact that c2d will bottleneck 7850 big time. Hell, it even bottlenecks my 4850 even at under-clock in the new games.
Upgrading to Ivy or sandy bridge will cost too much because I would have to buy another Mobo. So, if I buy a C2Q (used) and moderately over-clock it with a relatively cheap cooling solution will I still get bottlenecks then?

Thanks in advance.

At stock a Yorkfield QX9650 is only a shade slower than an Sandy Bridge i2400s is games. Thus I shouldn't think that a lower end Yorkfield quad with a modertaye overclock would be too bad a solution. Even the first gen QX6850 is about on a par with the i5-661.