I know the 915 chipsets only support Pentium 4s, but I believe the 945 chipsets all support Pentium Ds (dual core, as you wish). You should be able to use any of them. I would avoid the 955 & 965 models, however, because of the extremely high power output (thermal). I would say go for a 930, 940, or 950. You can find a bunch on eBay, I'm sure.
Some 945 chipsets support Core 2 processors, however, since your computer is a Dell, I would not bet on it. That being said, I would advise your friend to buy a new MB and CPU (and ram, if warranted). He will see a much greater performance difference by going to a C2D (plus the PDs are power hogs, and the original PSU might not like that - depending on other components).
I have a HP with and Asus 945G board. HP support said its board can go up to a 3.2 Either P4, Pentium D 9xx/8xx. I just ordered a Pentium D 920. It will be in Wednesday. we'll see.
I went with the PD 920 just to tide me over for 6 months or so. I will be buying a new mobo around feb/march of 08. I need a better CPU now. As cheap as the PD 9xx's are on ebay, i couldn't justify spending money on both a mobo and processor. Hopefully when i am ready in early summer, i can get a Core 2 Duo 6750 or Core 2 Quad cheaper than they are now. Maybe even one of the rumored 45nm or 55nm Intel dual cores.
If he has the money for a new mobo and time to reinstall his software, i would go with a C2D + new mobo.
He could go with a Pentium D 805 and OC it a bit. Those ran cooler and when OC'ed don't use as much as a PD 9xx and kill anything in the same GHz range. But there is no support for a C2 which would have been the better choice but not for his chipset.
Uh, I'm pretty sure Pentium D 915s run cooler than the 805's because one's 90nm and the other is 65nm. Also, Dells cannot, I repeat, cannot overclock. Even with a volt and FSB mod, the crap HSF design will cause overheating issues and the power supply will probably not hold the OC no matter how much Dell underrates their PSUs.
You'll be fine at stock with a Pentium D 915 and a 8600gt though.
Good choice. Hope it lives up to your needs/expectations. 43°C is not a bad temperature idle. Please post your loaded temperature if you are concerned/inexperienced. Everest seems to be the gold standard of PC monitoring, if you download Everest Home edition you will undoubtedly receive accurate information about the conditions of your computer (temperature, clock rate, etc). Everest Ultimate can also be downloaded from Lavasys' website, but it will be a trial version of the full-fledged program (available for a couple Jacksons).