The p45 chipset is the current chipset of choise for socket 775 motherboards, these are an intell chipset with crossfire support (ati's version of sli, although now ati uses internal links I think it's a bit better the sli). you can still use nvidia cards on a crossfire motherboard, just not 2 linked together.
I'm not up on which are the best p45 motherboards, after a quick look the Asus P5Q Pro looks good http://www.scan.co.uk/Product.aspx?WebProductId=818514
I'd stick with a ddr2 memory motherboard, as ddr3 is still a bit too expensive and the small boost you get from the extra speed is not really worth it. (the performance of faster memory tends to fall off quickly after it runs quicker than the FSB)