Spec me a CPU to replace the Q6600

[nippynoo]

Looking to upgrade a couple of home PCs, for under £300 each, with new CPU + M/B . But I'm completely out of touch with the latest families of processors etc and need some help. Last time I upgraded a couple of years ago I got Q6600's so that was quite easy, as it was the obvious "best-value" choice. But technology has moved on and now there are more choices..

(1) So which CPU?
I value quietness, so lower the CPU nm, the better I guess, for power consumption/fan noise. But I need ultra-performance (occasionally), so quad-core CPU maybe?

(2) I've got stack of old memory - can I re-use that? Think its DDR2

(3) And which M/B?

(4) And lastly, will I see much of a speed improvement over a Q6600?

thanks

 

[nippynoo]

..using Abit Fata1ity F190HD m/bs, not sure about the Q6600 stepping, will check later - not overclocked in any way (wanted to keep running temperature as low as poss). (I like quietness) (Hmm could add water cooling I guess?)

..use the sytems mainly for 70% general office, 20% games, 10% number crunching (CAD/CAM)

So do I stick with what I've got, or change? Boot-ups times to be getting slower and slower. I like what I've read so far about the 35nm i5.. and I like to have at least one decent PC in the house..

Thanks again