hunter315 :
Not quite, you can upgrade it to any Socket 775 CPU with an 800 MHz FSB that Dell chose to add BIOS support for, that is going to be a very very limited number. I don't think they ever support pentium Ds, i think the best you will get up to is a P4 with hyper threading in that machine, and for the price of the builds posted above i would suggest going for one of them rather than spending close to that much on a CPU that won't help the old dell much since most of the slow you are experiencing will likely be due to the hard drive being incredibly slow due to older tech.
Well that is true, but as you said this is going to be a very limited. In fact, unless they used the same motherboard for multiple systems with different CPUs it will be extremely limited. However, I don't think Dell says which CPUs are supported so finding out will be hard.
Only reason I am not telling him to go for a better build, is because I think he probably would if he could first. I mean, who on earth would ever want to use a Netburst based system?
But also because those old CPUs are really cheap now. For the risk of waisting like $5-$15, though probably closer to $5, and a chance it will work. It might not, but still pretty cheap to try.
And yea the HDD is a slow down also but remember IDE went up close to SATA 1 speeds, and on much better systems then Penitum 4 so he still will likely see a considerable boost.