OK, I've actually owned a recent HP and a new DELL 4700.
First off, I'll answer your question about noise.
My DELL is WAY quieter than my HP. My DELL 4700 uses single fan cooling (cpu is cooled my rear intake case fan) Where as the HP uses a standerd heatsink with its own fan and NO exhaust at all. The HP isn't loud by anymeans, but if you put them side by side there is a noticable difference. I'm kinda bored now, so I'll elaborate a little on my experiences with the two brands
Dells and HPs are very different computers.
1. Dell uses ALL custom parts that ONLY work will DELL and if you need parts you have to get them from DELL, where HP uses standard stuff and can be upgraded MUCH easier
2. HP are usually very small cases with bad airflo and cramped spacing. THis makes adding video cards and such very hard. DELLs are generally bigger, but lack airflo as well. Boths brands require aftermarket cooling if you add high-heat components like video cards. (I had a X800 XL in my DELL and it got WAY too hot)
3.Customer service was much easier to access with DELL and the DELL warrently was a full 2 years where the HP was only like 9 months. After being on hold for 30 minutes with HP I got a tech that proved to be helpful, but the couple times I called DELL I was talking to a person almost instantly.
Both of my prebuilt systems are still running strong but I strongly suggest doing a custom build. My HP and DELL can't hold a flame to my P180 custom build