Upgrading Motherboard and CPU?

Ok this is a re-post of a question earlier but this time I'll include a little more info.

My current setup is:

Motherboard: ASUS M2N-SliDelux

CPU: AMD Athlon64 X2 4800+

RAM: 2x 1GB kingston Hyper DDR2

GFX: GeForce 8800 GTS 640Mb

and my current PSU is 500w.

So.... I'm thinking of upgrading this and I would like to get a quad core cpu.

But at the moment my motherboard only supports dual so..... can anyone recommend a quad core CPU and supporting motherboard.

My budget for the Motherboard+CPU is around $400 but I could go over a little if required.

I don't really mind whether its AMD or Intel but I have heard Intel is starting take

I am also looking at getting a Gigabyte GeForce GTX 275 896MB, is a price of 267.477 USD any good for this card?