I built a system about 2 years ago with an Intel 945 psn motherboard and a pentium d 820 processor. it runs fine with 2 gigs of ram and an nvidia 8800 gts video card on it. I would like to upgrade my processor and dont want to have to reinstall windows because I have way too much stuff on my pc. my best bet is to upgrade to one of the processors that the motherboard can accept correct? I know that I can upgrade to the better pentium d's such as he pentium d 950 which is going for about $140 now...
1) is worth it the price  
2) will I notice any difference at all with an upgrade from a pentium d 820 to a 950..I play lots of games and watch videos and burn dvds and stuff mostly.
3) if i make this upgrade it should not affect my windows leaving everything intact correct?
4)what is my best option for an upgrade while leaving my os intact?
 
thanks so much in advance for any advice

I'd get a new motherboard and a C2D. See Tom's charts for the benefits of a 940 over an 820. Not many benefits at all:
http://www23.tomshardware.com/cpu_ [...] &chart=421

So, I'd recommend at least a C2D E4500 or higher. The comparisons in new games like Supreme Commander between a high end Allendale or Conroe vs. a Pentium D are clear. The only Allendale CPU's the Pentium D 940 beats are the new "Pentiums" (i.e. E2140). The E4300 beats it hands down.

http://www23.tomshardware.com/cpu_ [...] &chart=421

It would cost you more, but it would be worth it in the long run.

yes
 
the d will clock to 4ghz and its a much faster processor
 
or find a EE965 4.4ghz (16 mutiplier) or 955

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